It is for this reason that one may walk even one amah outside a city. The halacha is that half a shuir is always forbidden – even min haTorah. For example, one may not eat even a fraction of a k’zayis of pig.
So why, then, may one walk one amah outside of a city? The answer is because the halacha of not performing half a prohibition only applies to the first type of shiur whereby every fraction is part of the prohibition. It does not apply to the second type of shiur.
The Kiryas Sefer maintains that only the first type of shiurim do not apply to Bnei Noach. The second type does apply to them.
Based on this distinction, we can argue that becoming bar mitzvah is a shuir that applies to Bnei Noach as well – as it is of the second type of shiur we discussed.