Hi Hannah
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.
At 24 you would probably be the youngest on the tour, only you can decide if this worries you. Most people will be in the 50-70 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.
You'll see from the holiday description what kind of holiday is on offer with JY in terms of hotel accommodation used, activities undertaken etc. If you want 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 biggest. 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.
I've been to the US a load of times (not with JY), and it's a great country, with very friendly people, and I'm sure you'll enjoy your visit whichever holiday company you use.
Stuart