Bill's reaction was all the Doctor needed. He smiled to himself and chuckled, letting the topic rest.
"Macaroons are different!" came the Doctor's argument. "Macaroons are sweet clouds of rainbow dust, and you can't tell me otherwise. They're my latest obsession, and be thankful. It could've been fishfingers and custard!"
To respond to Bill's question, he became thoughtful as he spoke. "Of course. It was what I needed to go through at the time. I'm glad I got it out of my system, though. But yes, I looked like a 12 year old with no eyebrows, and had a thing for fezzes and bow ties." He considered River Song, and he softly smiled. "I know. She's remarkable. But she hasn't met all my regenerations, only my last four."
He then focused on Bill. "You've got a bit more warning than most. A few of my traveling companions didn't even know I could do that. Regenerate. You're not the one. It's not a topic I like getting into."