Board Thread:The Reference Desk/@comment-4138948-20150320112222/@comment-188432-20150326035226

Neither the novelisation nor the serial have much to say about the event, really. It's just a comic gag. He's turned into a baby in one scene, fed baby food in another, then restored to adulthood wearing a nappy in the final scene of the story. No discussion of his mental acumen is given.

It's possible that you're remembering what happened to the other temporal victim in the story, Stuart Hyde. Perhaps through bad acting, he doesn't seem to change that much, mentally, when he ages. So he's in the apparent awkward position of believing in his mind that he's 27 when his body is much older. The book, too, doesn't suggest in any way that he's lost any mental acumen, though he does believe after the initial transformation that he's got a hangover. But in any case, there's no indication whatever that the villains of the piece are actively tormenting him by keeping him in any particular mental condition.