Hi
This is a hard question to answer, as it varies form holiday to holiday, but you would clearly be one of the youngest, if not the youngest.
In my experience, there are usually a couple of people around the 35/40 age group (or younger), possibly more. The majority of JY customers are in the 55/75 age range, with around 70% being women. There are a number of threads on this topic, and if you look back you'll find them pretty easily.
Having said all of this, age is simply a number, and I've met some very "young" people in their 70's or 80's and some very "old" people in their 30's.
If there are a couple of holidays you're interested in Ben can let you know the number of people say under 40 already booked, which might be of help.
Good luck.
Stuart