I am analyzing cognitive performance and training effects on a cohort of healthy people with increasing age from 18 years. Is there any consensus paper defining age-cutoffs for grouping people into young, old? How do you define your groups?
In addition to what Nicola said, I would also consider education and sex as possible variables, specially in adults. For example, you could divide your groups by sex and by years of education (0-8; 9-12; 13-16; 17-21). Then you could run different statistics to see if there are any differences and cluster your groups by either age or education or both!
When analyzing or comparing motor abilities/neuromuscular function, we usually use non-scientific age-related classification based on how they are divided in most sports (U9, U11, U13, U15, U17...)...