Toddlers: 3-4
Children: 8-10
Teens: 16-18
Young adults: 21-40
Adults: 45-65
Elders: 65+
I agree with @Simmingal that people in their 40s are definitely beyond young adulthood in the sense that they're usually firmly settled/established, whereas people in their 30s, at least these days, are usually still in the process of settling down/making a name for themselves. My sister is 34 and still seems young to me.
I usually think of adult Sims as being 45-ish, because it's firmly middle age and most people will have obvious signs of ageing by then (no offence to any 45 year olds!). In any case, Adult Sims seem way too mature/wrinkled to be anything under 40 imo.
60 is still middle age to me - I have aunts/uncles in their early 60s and they definitely don't seem elderly to me. They're not even old enough to claim a state pension yet.