I think it sounds like a great idea!
I traveled round the world on my own, yes I am only 22, but I met people from the age of 18 up to 67 that traveled alone. To me age makes no difference as long as you get on with each other. I mean, we all have 1 thing in common, traveling.
Asia is a great place to start as well, nice people, it's cheap, amazing food and beautiful scenery.
As for the hostels, it is mostly young people but like someone else wrote, people who stay in hostels are open and welcoming (most of the time).
If you find that you don't like the hole hostel thing the hotels are really cheap around Asia anyway.
I really hope you decide to go, you wont regret it. If you come down to Phuket send me a message and I'll show you around a bit if you like. (When I'm of work, that is.)
[ Edit: Sorry, no promos please. ]