Hi Nicholas
If you look at some of the other threads on this board, you'll see a lot of discussion about the typical age mix on JY holidays, and also the male/female ratio (usually women outnumber men 3 to 1).
At 28 you would probably be one of the youngest (probably the youngest) on most holidays, only you can decide if this worries you. It does seem that younger people are using JY more and more, but it's still the case that the vast majority of people will be in the 45-70 age range, with a few either side. Age isn't a very good indicator of people's attitude to life, and there are some very "young" people in their 80's, and very "old" people in their 30's.
In my experience there will always be a group of people who meet up in the bar after dinner for drinks/chat, and this won't always be the youngest ones. If you want a holiday with heavy drinking, loads of time spent in clubs/on the beach/around a pool, then JY is the wrong holiday for you.
I think it really depends on what type of holiday you're after. If you're looking for a more active/outward bound type holiday (ie. camping, treking, white water rafting etc) then there are companies that offer this, Trek America being the classic example in the US, "The Adventure Company" being another that operates worldwide, plus numerous other companies operating primarily in the Indian sub continent. This type of holiday will have lots of single people, although they cater for couples/groups as well, so it would not be an exclusively singles holiday. It's really up to you.
You'll see from the holiday description what kind of holiday is on offer with JY in terms of hotel accommodation used, pace of holiday, activities undertaken etc. Don't assume that the long haul holidays will have a younger age profile, because in my experience it's mostly retired people who can take 4 week holidays timewise.
Good luck
Stuart