What age was the ‘older’ boy?

Abuse or experimenting? It could have been a bit of both.

The reason it took so long before it began to bother you is because your body and your mind work to protect you from stuff that may be hard to deal with.

It is a good sign that you are thinking about this now, it means that you are strong enough now in mind and body to deal with the memory.

And the way to deal with it is to accept that it happened, it might have been a mistake, you might have been quite foolish, you might have been used, but it’s all over now and whatever your mind made of it at the time, it’s now time to heal, to let it go and enjoy the rest of your life.