Babies: 0-2 months (they only lie on their backs so can't be any older than that)
Toddlers: 1-3 (they learn to walk, talk, eat by themselves)
Children: 6-11 (they go to school, so they can't be any younger than that, also they can't be any older based on the way their body looks)
Teenagers: 12-18 (some teenagers really are that tall at 12, so I go with 12. But based on their behaviour I'd say they are about 15-18)
Young Adults: 19-39 (some people still look youngish in their late thirties)
Adults: 40-64 (not so young looking but not quite old yet)
Elders: 65+ (wrinkles, grey hairs can't be hidden and disguised anymore)