Do you mean the youngest age at which it is possible?
I had a series of mutually consentual sexual encounters with a 2nd-grade classmate. I wasn’t exactly sure what we were doing, but I knew it was both naughty and exciting to get nekked and bump and grind with a girl. It took a couple of attempts for me to figure out how to put tab A into slot B, but my equipment was certainly ready and able, and I somehow knew that I was supposed to put a tab in a slot… or something. And when I did figure it out, the sense of accomplishment was quite rewarding.
So, bottom line, the physical ability and instincts are there even in the prepubescent.
Now, if you mean the youngest age at which an adult should be permitted to engage in such an affair with a minor, I would have to object to making the standard a chronological age and suggest instead that psychological age be considered first.
And the younger party should be mature enough to regard his or her partner as a peer, not as a parental figure or superior.