Carl was the son of an important person on his colony. When the colony was destroyed, Carl survived in an underground bunker, where he was provided for by House and spent his days learning lessons with the other children.
Because no one remained to reclaim the children, they continued their lives in the bunker, ageing physically but not mentally. They still saw themselves as children.
When the Sixth Doctor and Peri tried to rescue Carl and Roger from the murderous House, Carl didn't want to leave because he was afraid. Roger stayed with him as the Doctor and Peri left in the TARDIS. (PROSE: House)