Would it have been fine if he were older? Really?
For every year you make him older and wiser, the creepier the fact that he "cluelessly" opens by offering marriage to someone his soldiers kidnapped to offer someone else as a slave, just because she's pretty (doubly-so with the implication seemingly hanging there that he might otherwise have been like 'yeah, sure, send her sounds great'). Like, the plot tries to slight-of-hand it by saying "the place they were planning to send her was so much worse. so he's saving her, right???" and by having the attendant offer her freedom in desperation, but the fact remains that the protagonist for his own part effectively telegraphed "hey lady we captured, you're pretty, how about you marry me instead of being a slave to that other guy".
Basically, if we're looking at it objectively, the whole thing's fucked top to bottom.
Honestly if it were IRL, the age gap still seems like the lesser evil to me. 14 is certainly old enough to fall in love (or "love"); we can imagine this happening and hell, it probably more-or-less did at some points in history, at least in spirit if not in the precise details. He's a prince who can apparently make unilateral policy decisions on at least minor issues in a country where royalty randomly enslaving people on a whim (at least mermaids, which is what's relevant here) appears to be considered above-board, so the power-imbalance is sickeningly in his favour despite how much younger he might be. Worth considering: why age-gaps are considered bad? (Two wrongs don't make a right, tho)