I think the age thing is different in each countries : in France you can have sex at 15, and you can marry at 16 (if I am not mistaken) with your parents agreement.
Sanji is 21 and Pudding 16. Yes there is indeed an age difference, not a huge one because Sanji is still quite young himself. The difference is not shocking, but I think Oda won't take risks and if he goes for this ship he won't ship them before Pudding grows up a little.