If it's okay, Healed84...

I do hope you bring this up with your therapist. You were too young to make a conscious decision to engage in anything sexual. They took advantage of you. The fact they were older makes it even worse. They were so much more mature, and you were innocent. This was, in no way, any fault of your own.

As it was already pointed out, the fact you do not remember - yet the event comes back to haunt you, all the same - really points to this troubling you more than you might realize. Please talk with your T about it.