I really wouldn't worry about it at all. You took time out to concentrate on your health and now you're back in education and in, hopefully, a stronger frame mentally. This will stand you in good stead, certainly.
By the time you get to college and university, you'll realise that the age distinction quickly widens. You don't even expect people to be the same age as you but rather of all ages. At college, there's a lot of older students retaking and some coming back into education after a spell out and at university, well, you'll see people in their twenties, thirties, forties and beyond! No-one really cares and when the difference is as slight as yours, 2 years, no-one even notices.
I think the most important thing is to just work hard now and get back into the education and learning frame of mind. Do the best you can and actually make the most of the fact that you're in a stronger position than most of your counterparts because you're more mature than them, more determined, more knowledgeable. It might seem like a long road ahead, and sure it is, but you'll quickly start to enjoy it once you get back into the swing of things and you know it's definitely worth the effort.
It's really not an issue at all, so try not to let it get you down. Hopefully this will come in time. I'm glad you're not letting ill health deter you from your education and are coming back to it with a view to looking forward towards university. It's a really good, positive attitude to have and let it drive you forward. Once you get going after the first couple of weeks of thinking "Oh, I'm older than these guys" you'll realise you were making a little fuss over nothing.
All the best