Note these ranges are not proportional to their sims lifespan, just the equivalent age range as I see it
Babies 0-6mos
Toddlers 18mos - 4.5 years
Children 8-10 years
Teens 15-18 years
Young Adult 19-30
Adult 30-60
Elder 75+
Really the gaps don't bother me that much, and Teens being as tall as adults has been my reality since I became an adult (because I'm short) and I've known teens I thought were adults until I found out later. (nothing weird, there was just this one guy in my kid's drama club in HS that I thought was an Adult helping out, rather than a student) So that doesn't bother me either.