Here is how I perceive the different age groups. This based on the sims appearance and abilities, not the number of days in game.
Baby: 0 to 5 months
infant : 5 months to 2,5 years
Toddlers 2,5 to 6 years
children : 6 to 12 years
Teens : 15 to 18 years
Young adults : 18 to 40
Adults : 40 to 65
Elders : 65+
Note on teens. I left a bit of a gap between children and teens on purpose because that is how I experience aging in the sims at the moment. I think the teens in sims 4 are too tall and can do to much adult activities to pass as 12 to 14 year olds.
65 used to be the official age to retire around here for years (now 67) so I think that change in life seems fitting for the beginning of the elder lifestage but I think it can vary depending on the sim.
