My Alien daddies.
That's exactly what happened, they came, aliens came, they took all of us in their giant spaceship out back to the mother planet.
We thought we are so smart, that we'll get another chance to ruin their planet, except they were much larger than us, much smarter. Their planet, it wasn't meant for us, it was meant for them, for us to be able to adapt and live with them, they had us regress to little kids again, we needed their attention and care.
Grown-ups, full adults, once they make it back to mother planet, they regress back to baby ages, their body would shrink, they still might look like teenagers or pre-teen, but their mind would be same as a baby.