15: able to get a job, usually have reached or are really close to reaching their adult height. If monsters survive this long, then they'll most likely have a long life, so this age is celebrated

16: able to sleep with people who are 20+ years old without the adults getting pedo charges, also able to drink, do drugs, etc.

18: not that different from 16, half teen half adult, usually just figuring out their life

20: officially an adult, usually the age when most monster college students apply for college, if they date anyone who's 15 or younger they'll get arrested

these rules don't apply for cat/dog monsters, once they're 13 they're allowed to do pretty much anything thanks to the weird way that cats and dogs age irl