It depends from where your ship is sailing. Here is your answer straight from Celebrity's FAQs on their website (link).
"The minimum drinking age for all alcoholic beverages on Celebrity Cruises® ships sailing from North America is 21. The minimum drinking age for all alcoholic beverages on Celebrity Cruises® ships sailing from South America, Europe, Asia, Australia and New Zealand is 18. Guests may contact Guest Relations or refer to Celebrity Today for specific minimum drinking age information on their cruise vacation."