Hi Tom,
With JY you will probably find a mixed bag of ages with the majority likely to be in their 50's and 60's. You may find someone in their 20's/30's but its not certain. Both Holidays I've done ages have ranged from 20's to 70's but if you are looking for a group of people at the younger end I doubt you'll find it.
Saying that from my experience everyone mixes really well irrespective of age so if been around a few older people holiday isn't a problem to you why not give it a go and have a great holiday.
Mick