Originally Posted by
Drops Of Venus
There isn't a 100% certain answer to this, because Marvel never actually bothered to explain how exactly Billy and Tommy Maximoff became Billy Kaplan and Tommy Shepherd. Allan Heinberg originally wanted them to be THE Billy and Tommy Maximoff, explaining that they survived somehow and were adopted by the Kaplans and Shepherds, but they ended up not going in this direction, so the official canon is that Wiccan and Speed are just described as reincarnations of Wanda's original children, although there isn't any more clear explanation about how the reincarnation process happened and how it makes sense, chronologically wise. The only writer who came close to explain that was James Robinson on his Scarlet Witch series, where he raises two possibilities about the origins of Billy and Tommy (those explained by Journey on the last page). Al Ewing also wrote Billy referring to himself as a ''retro-reincarnation'' during his New Avengers run, implying time-travel was involved in his origin.
But then they would have to worry about other stuff, like the fact that the heroes would age out of their prime very quickly, leading to possible deaths or retirement of many characters. That would essentially force Marvel to stop publishing some of their most iconic characters, which I don't think was ever a viable option for them. I mean, even Billy and Teddy would be affected by that, since they would now be in their 30s and would probably never be allowed to be Young Avengers again. Sliding timescale actually gave the team an opportunity to exist for longer without things getting weird.