Wait he’s still 16…

Actually, he's 17 - almost 18.

Eren 6 years old (Pg. 11) + 1 year (pg. 15) + ten years (pg. 36 & 39) = 17; plus "early birthday present" (pg. 38) - so he's probably only a few weeks shy of 18; and if they had been a couple near the same age with each other not too many people would have an issue with that (especially given the fact that he's the 'top')!