>>4238
If you have a society where multiple races, each from their own planet, with their own evolution and biology, this is kind of inevitable.
Not every species matures at the same rate, mentally or physically. And that's without even touching on what may be considered acceptable within each culture.
Consider for example if dogs where sentient and integrated into human society, would they be of legal age at age 2 or age 18?
The real horror show comes when you consider species with biologies that essentially don't have childhoods. Like an alien species that lays hundreds to thousands of fertilised eggs at a time and then abandon them to nature, the eggs hatching nonsentient versions of their parents who latter develop sentience as they grow, with the parents readily eating their own young if an opportunity presents itself. Or the problems you get when a female sentient mantis takes a human it has met at a bar out for a one night stand…
All of these are fairly common behaviours among various animals.
Cardassians using debates as foreplay or klingons using fistfights that way, is down right pedestrian by comparison.
Hell, what do you do if sexual dimorphism in a species has produced females who are dramatically less intelligent than the males, and are little smarter than dogs. Do you make those females full citizens of your country too?
The least of the issues in trying to make a multispecies interstellar empire, would be worrying about the when someone is considered adult. The easiest answer would be medieval french law, where each ethnic group in the country essentially got to make their own laws instead of holding everyone to the exact same standard.