See Jackson Laboratory web site

(http://www.jax.org/staff/harrison/labsite/ger1vLifespan1.html#VD).

“…most studies focus on two or three specific life phases: mature adult, middle age, and old. Figure V.3 illustrates these life phases using a survival curve that is based on a large cohort of C57BL/6J mice (150 males and 150 females). 
The mature adult group (3–6 months of age) is the reference for any age change, whether the change is developmental, maturational, or senescent. This group consists of mice that are past development but not yet affected by senescence. For the mature adult group, mice should be at least three months old because, although they are sexually mature by 35 days, relatively rapid maturational growth continues for most biological processes and structures until about three months. The upper age range for the mature adult group is typically about six months. Past this age, mice might exhibit some age-related change”.

In short:

Infants

Mice are considered infants from birth to 21 days of age. At that age they are weaned (prevented from nursing)—their mothers are removed from the cages

Juveniles

Mice are considered juvenile (young) from 3 weeks to 8 weeks old.

Adults

After 2 months, mice are considered adults. Laboratory mice live about 1.5 years.

On the CNS level of mammal organism you would really need to determine what is considered to be adult for mice, because as far as I know adulthood is really a complex process, that depends not only on strain of mice, but also on sex and probably other factors. There are some scientific and some I would call half-scientific databases providing basic information about mice, for example (where i took data from): http://science.education.nih.gov/supplements/nih4/Energy/activities/508/mice-ref/mice_ref.htm

or book on Biology of the Laboratory Mouse ed. Earl L. Green http://www.informatics.jax.org/greenbook/index.shtml

But after short search through PubMed I found that for frog Rana pipiens there are some differences in juvenile vs adult CNS (http://www.ncbi.nlm.nih.gov/pubmed/8071657), although their biology is really different from mice. So to conclude either I was not lucky to find any data on this topic, or there is no data on this particular problem.

I agree. When we ordered adult mice for experiments we always specified ~6 weeks of age. The 8 weeks of age mice are definitely adults.

Mice or rats after two months are considered young adult ( 2 to 6 months are young adult) after 6 months are considered adult rodents (mice & rats). Back to question, which type of experiment are you going to do in order to use animals as young adult or adult or aged adult.

I think it depends on what system you are looking at. For example, I believe "adult" has a different range when looking at immune function compared to, say, bone growth.

90 days is reasonable cut-off, in mi experience

I agree with Bela Volgyi and Michael Pecaut. 8 weeks old animals should be considered as adults. We work with brain slices and need matured brain to study neuronal excitability, as we know GABA is main inhibitory neurotransmitter in brain. However, in new born action of GABA is excitatory. Thus, we prefer more than 3 week old animals, by then brain is considered to be developed).

good luck

See Jackson Laboratory web site

(http://www.jax.org/staff/harrison/labsite/ger1vLifespan1.html#VD).

“…most studies focus on two or three specific life phases: mature adult, middle age, and old. Figure V.3 illustrates these life phases using a survival curve that is based on a large cohort of C57BL/6J mice (150 males and 150 females). 
The mature adult group (3–6 months of age) is the reference for any age change, whether the change is developmental, maturational, or senescent. This group consists of mice that are past development but not yet affected by senescence. For the mature adult group, mice should be at least three months old because, although they are sexually mature by 35 days, relatively rapid maturational growth continues for most biological processes and structures until about three months. The upper age range for the mature adult group is typically about six months. Past this age, mice might exhibit some age-related change”.

Dear Stefano,

I also agree with 8 weeks as young adult mice. In this publication from the lab where I did my diploma thesis a colleague compared P4 and P60 rats in the development of the auditory system. http://www.sciencedirect.com/science/article/pii/S1874391912007725#

I also agree that it depends a little bit what you want to do. If you´re doing primary cultures or slice experiments sometimes it is not possible to wait until 8 weeks because the cells do not attache anymore or the slices are not surviving the slicing process.

I think for most experiments 8 weeks is fine.

Best luck,

Claus

Hi Stefano,

You may find this link useful: http://research.jax.org/faculty/harrison/ger1vLifespan1.html

Take a look at the section: How we define the life history phases of mice; how they compare with humans

Arbitrary but defensible? I looked at this a couple of years ago after being deeply frustrated with the MGI definition, or lack there of. The notion that we had was that although nobody will actually agree on a date, for the purposes of labeling data we needed to create some date that was, arbitrary but defensible.

from my blog:

"According to the work of Finlay and Darlington (Science, 268:1578-84) with the chronometry of species, the final important steps in brain development of mice occur 29.7 days after conception, or postnatal day 12 (birth is P0 in this case), menstruation typically begins between postnatal day 25 and 40 and body growth is completed at about age postnatal day 50. So we can use the arbitrary date of postnatal day 50 as the definition of adult mouse, as this is a reasonable standard for an adult. We will define the day of birth as postnatal day 0. Mice between the age of P0 and P24 will be termed juvenile and mice between P25 and P49 should be termed early adult."

The full rant is available here:

http://blog.neuinfo.org/index.php/news-events/general-information/define-adult

Other data that you may find useful: AnAge database from Olivier Toussaint's group

http://genomics.senescence.info/species/

Here'a a nice book on the topic "Neurobiological Aspects of Maturation and Aging":

http://www.amazon.com/Neurobiological-Aspects-Maturation-Progress-Research/dp/0444411305

I would say 90 days would be young adults, just like Ricardo Pautassi mentioned. If you want to use adult rodents, 6 mo is the cut off, in my experience.

Hi Stefano,'

I asked the almost the same questions differently to Jackson Lab about the difference in size of brain of 6 and 16 week old C57 mice. They said that 4 weeks old mice are having about 5-7% smaller size than the 16 weeks old ones. Here are some useful links regarding the weight and size of the brain in different strain and their maturation timeline.

http://phenome.jax.org/db/qp?rtn=views/catlines&keymeas=22701&ustrainids=&projhint=Jaxpheno2

http://phenome.jax.org/db/q?rtn=meas/catlister&req=Dbrainqqq9

Hope this will help

Good Luck

IMO, rats 60 days of age still show signs of hyperactivity, playfulness and overall novelty seeking typically found in adolescents. After day 80 to 90 they are more quiet and lack the above described features.

6-8 weeks

The following article can help you: (published in Life Sciences in 2016)

http://www.sciencedirect.com/science/article/pii/S0024320515300527

https://www.researchgate.net/publication/283841162_Men_and_mice_Relating_their_ages

Article Men and mice: Relating their ages

Adulthood is biologically defined as the age at which a human or other organism reaches sexual maturity. Mice become of mating age sometime between 6 and 8 weeks of age.

8 weeks is enough for considering mice as adult.

How about the sperm count for males if it can be used for estimating age on rodents

Adult mouse - A mouse 8 weeks to 2 years old, with the day of birth defined as day 0.

• Aged mouse - A mouse 2 to 3 years old.
• Mature mouse - A mouse 3 to 6 months old, with the day of birth defined as day 0.
• Middle adult mouse - A mouse 10 to 14 months old, with the day of birth defined as day 0.
• Old mouse - A mouse 18 months to 2 years old.
• Pubescent mouse - A mouse at the age of sexual maturity, generally 28 to 49 days old.
• https://confluence.crbs.ucsd.edu/display/NIF/NIF+Annotation+Standards+for+Age+Classification
• I add my mice's life spans:

Adult mouse of 6-20 weeks old range are the best to carryout studies. For more details refer the below link Article Does Age Matter? The Impact of Rodent Age on Study Outcomes

This is a question that cannot be answered as asked.

Different CNS systems mature at different times. In general, subcortical nuclei mature first, followed by telencephalic structures. Connections lag behind. While brainstem and

6-8 weeks is enough for considering mice as adult.

At what stage do we consider the mouse brain as adolescent brain? around p6-p18?

8 weeks is adult, it may be not for every single system but is adult. We start using them for inflammation models from 6 weeks of age