Mike,
True the majority of travellers will likely be in their 50's & 60's but from my JY trips so far ages have ranged from mid 20's and 80.
Saying that you would likely be the youngest Man from what I've seen, Men seem to start later than the ladies sort of your age upwards.
Think the holiday you choose will have a bearing on the group make up.
Done a couple of america trips and the age range was a good spread.
Andalucia in November edged more towards the older end (apparently because its cooler someone commented), also a christmas hol i did by coach eemed to attract a older group.
Majorca at easter a couple of years ago was a good mix also.
I've enjoyed all the holidays and have mixed with people older and younger (I'm 54) without a problem, just had a great time.
Got two more booked this year, Austria in august and route 66 in October.
Mick