on some cruise lines, "underage" passengers have an "underage comment" on their "cruise card", sometimes visible sometimes in the magnetic strip, so they cannot order / sign for alcoholic beverages.
Also, Lorenzo, please keep in mind that some cruise lines will not accept passengers under the age of 21 unless they are accompanied by somebody 25 or older. As you are married, this should not apply to you, however. But it's always better to check.
Regarding what cruise to take, it really depends. There are lots of cruise lines, lots of destinations etc.
Tell us a little bit more about what you and your wife are looking for when taking a cruise (and probably also what you are not
looking for) and we should be able to give you some recommendations. Also, what destinations are you interested in?