Yes you are likely to be the youngest but dont let that put you off, life isn't a rehearsal if you want to do it, go for it.
27 is the youngest I've seen on two of the JY holidays I've done, thats out of 5 trips, 2 of the other trips youngest was in their 30's while it was mid 40's on my christmas hol (don't know if that is relevant).
Don't let age put you off people do mix well, I've socialise on these trips with the youngest and the oldest with no problem.
Mick