I think it’d be more accurate to say “imprecise” rather than “arbitrary.” For example, the legal drinking age is set to 21 (or whatever) based on politicians’ estimation of when most people are mature enough to handle the consequences. It’s true that there is no way to exactly specify a perfect age limit, but that doesn’t mean the limit was set randomly without any reason or basis, i.e., arbitrarily. Sorry if I’m being too pedantic; it’s just one of those nights I suppose.
Hell, even legal age of consent is arbitrary (please don't argue this -- if it's not it won't vary across developed countries).
The absolute majority of legal lines we drew are arbitrary.