Not sure if this should rather belong to the head canon topic, but eh, here we go.
I believe this planet has its own laws of nature regarding people's appearance, possibly due to the planet's size and position in space and gravity thou art a heartless b— sorry I digress. They reach maturity faster and stay good-looking longer. They probably live longer as well if Bugenhagen is just your average senior citizen. So Cloud at 15 must have looked way less like a kid and more like a young adult, and Tseng bloomed even earlier than that. So acceleration is the key.