Just got back from the mexican riviera and they had no problem serving my 18 yr old son both at dinner and at the lido deck during a party. I think they use the Mexican legal age of 18 even though they won't say that. We sailed the Spirit, Dec18th.
Also we brought several bottles of wine in our checked luggage no problem. Our waiter did not charge us a corkage fee for 4 bottles he opened for us. I think they worry about you adjusting their tip if they chage you the corkage. Just my opinion.