Personally, I believe that I should tell them right in the beginning, before we even become official, because dating a person with trauma is never easy and I should leave the decision of whether or not they date someone with such experience IN THEIR HANDS.

I told my boyfriend that I was repeatedly assaulted two days before we started dating. He was extremely shocked, devastated and even cried. I observed that he is still going through some secondary traumatic stress. I feel bad for putting him in this situation, but I know he loves me and he will do whatever he could to help me through this.

You need to tell him, but at your own pace - only when you're comfortable with it. He has to know at some point because 1) you're in a relationship, 2) he should be trustworthy enough for you to share with him such an important piece of information and 3) he has to know about it to avoid triggering you physically/ making offensive jokes/ etc.

When you decide to tell him, it might take some time for him to process it, and it is totally fine - I'm sure you would understand that they didn't expect to hear that you have gone through something so traumatising, at such a tender age. But if he loves you, shortly after, he will show compassion and complete understanding.

Then, you'll know that he's worthy of your time and that he is mature enough to respect you and handle the situation.

N.B. To my definition, that certainly was NOT how you lost your virginity. Personally, I define losing virginity as they first time you have consensual sex with someone. Like you said, rape does not count.