Have not thought about it much. By the look of the sims the game gives an impression of reaching for the consumer group of shallow teenagers, since all the sims look very young and fresh, and teen sims almost like adults, and elders act very old. I liked the teen look in TS3 better, more distinguishable as younger than YA, which is how a teenager look to an adult in real life, despite their own ego..
Baby: <1 year old, because of size and gameplay
Toddler: 3 years approximately, given their skill development
Child: 5-10 years because of school and looks
Teenager: 12 mostly; in some cases up to 17 years; they look like YA, but act younger
Young Adult: 18 (legal adult age) -30 years or older; because it's the sim age where all gameplay is accessible, and looks
Adult: ca 35-55 years; few things change in their looks from YA stage, some gravitational drop and wrinkles, but nothing happens in gameplay
Elder: >75 years; their idles make them seem very old although they look so fresh