I am 34 years old, and still vacation with my parents. In their mind, "the more the merrier," they also travel with other couples. And since most of these vacations involve traveling to Disney destinations and my Mom is a CM, it usually ends up cheaper for us because of "friends & family" discounts. Of course, DH and I pay our own way. This does bother DH a little, so we also take our separate vacations, but my parents pretty much arrange it so they are traveling at the same time as someone else, even if they arrange everything separately.
My parents did pay for me to travel with them until I graduated from college. I was responsible for my souvenir money and snack money, but they paid airfare, hotel, most meals.