Romana only looks like an adult.
Four was 6 to 7 hundred years older than she was depending on which episodes you believe, and he was a sexually immature adolescent, which is why she must have been a fricking child compared to what he brought ot the party... then how did one have a grand daughter? i never liked the genetic loom idea, but i did like the idea that a gallifreyans first life is comparbale to a human beings, but the first regeneration makes them sexually retarded, and it takes them a few hundred years to redevelp those drives.