A couple of months ago I took my 18 and 20 yr old kids to a big water park. I kept pace with them and had a blast. Needless to say, I had aches and pains the next day but it was well worth it. They were surprised I could keep up.
Some kids in thier late teens and early 20's are so badly out of shape - too much computer time and not enough exercise I guess. I am not saying this applies to your kids in particular but your post made me think of this.
I often pass them while out cycling or hiking to do some skiing out of bounds - some of them look like they are going to pass out from overexercion. The only excuse I will accept for thier lack of apparent fitness would be severely hung over.