I have a 18 year old who has been on 3 carnival cruises. My now 12 year old does the camp carnival thing, but my 18 year old never has.
Her first cruise was when she was 16.
So we do a lot of stuff on the ship together. It truly changes the character of the trip. When my husband and I go alone we have a very relaxing time. But, with my daughter, we do every silly game they have.
On one ship they had an auction for families to buy stick horses to dress up and race the rest of the week. We didn't know about it right away and wondered why people were carrying these things around. We found out and went to the horse races, what a blast. The newly wed game is great. We play bingo everyday, and go to high tea. There's a lot of mother daughter bonding going on.
Also, if you are interested at 18 she is allowed in the casino.