I base it on my kids and our family LOL
Newborn 0 to 5 months (eat, sleep and poop)
Infant 6 months to 1 year (I based it on the age of crawling, standing up and eating foods)
Toddlers 1 year to 5 years (based on from when they walk and talk to just before starting "big" school)
Children 5 to 12 ( based on our primary school years that are for children)
Teens 13 to 17 (Based on secondary school years and when the parents start getting gray hairs)
Young adults 18 to 30 (My daughter is very self sufficient and in Uni at age 18, sims start Uni as young adults)
Adults 30 to 60 (I guess that is when the bills start and hopefully end for the next life stage to be care free)
Elders 60 plus (Now its time to relax and enjoy all the fruits of your years of labor)
