I’d do it a day later, since I was drunk as … something, when it happened, which was kind of sad. FYI I was 18 (barely 
) at the time. I still have contact with the girl, too. 
This is a bit of mental masturbation^H^H^H^H a thought expderiment that I have engaged in. I think that having the mental capacity and knowledge of an adult at five years old would have far ranging unintended consequences. You may find it impossible to behave as a normal child and have normal relationships with your peers. You’d be treated as a freak by other children and that would not be likely to change as you grew. This would change the dynamic of any relationship you remember from the first pass at life and in all likelyhood those relationships would not happen the second time around.
As for the specific questions I probably would change it. There are many situations where an inexperienced person should seed the services of a professional but this isn’t one of them.
I would have lost it “the first time” to the guy I dated back in High School. I was 16 and he was a very experienced 18 (yes, I caught the “bad boy” syndrome). The reason we didn’t at the time was the timing was all wrong. However, I caught up with him about 10 years later. He and I said we had always wondered what sex between us would have been like, so we planned a weekend together. It was INCREDIBLE!! I don’t know if it was the pent up tension between us, or just that he was that good, but…WOW! 
I can only imagine if my first time had been that good… Pardon me, is it hot in here?
The “second time” was when we were both 21. We were college best friends/sweethearts, engaged, and mutually lost it to each other. It wasn’t that spectacular. We were clumsy and embarrassed, but that made it all that more sweeter (come to think if it, we only had sex once). After a traumatic breakup, we have apologized to each other and remained friends, even though we married other people.