In the Sims 2-4 there age generally 7 life stages in the game. I have included the age ranges that these stages are considered to cover as well as the age they mostly represent.
Baby (0-1) Newborn
Toddler (1-4) 1
Child (5-12) 8-10
Teen (13-18) 16-17
Young Adult (18-40) 18-24
Adult (40-60) 40-50
Elder (60+) 80+
Now those are pretty big time jumps in age which miss out on many important aspects of life. Anybody with experience in the real world (which I assume is everyone unless there are some bots and basement-dwellers here) knows that there are big differences between the higher and lower ends of these age ranges.
What I propose are 11 life stages that will represent all aspects of the human lifespan
Baby (0-1) Feeding, changing, cuddling,
Toddler (1-3) Focus on early development such as walking, talking, potty training.
Preschooler (4-6) early social development, attending preschool, imagination.
Child (7-11) Elementary School, Homework, After School Activities, Pranks, Friends
Pre-Teen (12-14) Crushes, Pimples, Middle School, Mood Swings, Chores
Teen (15-17) High School, Part-Time Jobs, Dating
Young Adult (18-30) College, Marriage, Early Career
Adult (30-45) Parenting, Career
Middle Aged (45-60) Same as adult except looks older. Midlife Crisis
Elder (60-80) Retirement, Grandchildren, Declining Health
Geriatric (80+ Equivalent of a centenarian. Very few make it to this stage