Unfortunately, I don't see a way to do it without exposing our own traumas. Like, because I went through the wrong puberty, I grew up with a bone-deep certainty that I was an ugly unlovable freak that no one should look at or touch. I missed out on the relationship-related life milestones that all my peers got to experience, essentially rendering me developmentally stunted.
I think parents would care about that, I know mine did, but we didn't even know what was wrong. If they're legitimately worried about "regret" then make them know how much their child will regret not starting earlier.
And it's humiliating to even talk about and I know most people aren't going to want to do it, but I don't see another way.