Hi Ricardo,
The appropriate age for experimental mice is going to depend a lot on what the experiment is. For example, studies of early development need newborn mice, but studies of Alzheimer's disease need older adult mice. As a start, it might be helpful to look for some papers which use similar methods to what you're planning and see what age their mice are. If you use a disease model, you should check when the mice develop the disease and decide what disease stage you want to look at.
In general, most people consider mice to be "adults" by 2 months of age, though many studies use older adolescents/young adults around 1 month, or slightly more mature adults at 3-4 months. There's a weight chart by age for a common strain here if you want to see how they grow and gain weight as they mature:
https://www.jax.org/jax-mice-and-services/strain-data-sheet-pages/body-weight-chart-000664
You'll also need to consider any type of manipulation you will do (like special diets, drugs, or surgery) to make sure you have enough time. Some experiments take a long time or are hard to do in very young mice, which will put a limit on how young the mice can be when you observe the results.
thansk for your answer, yes actually I´m going to give some diets of prebiotics to them, and I'm going to measure some biochemical parameters like glucose trygicerides and others, and I was thinking about using an age between 8-12 for it , but with your answer I have a better idea about what to do