Atlanta Cruiser said it perfectly. Of course, there is a wide range of personalities and interests amongst 18-year-olds, and you know your daughter best.
On a 10-day cruise several years ago, we befriended a lovely couple who had brought their 18-year-old and 17-year-old sons. The boys were as different as night and day. The older son was a somber, middle-aged man in a boy's body who loved nothing more than sitting with us adults and drinking tea while discussing politics and current events. The younger son was a bon vivant who made friends his age immediately and we scarcely ever saw him, except for when he occasionally ran past with a gaggle of shrieking adolescents. Both boys loved the cruise!