"Baby Mycroft is just Mark Gatiss in a CGI reduction, he’s never been any different. [laughter]
I’m pretty sure that’s true of Mark as well, he was just a mini Mark in a three-piece suit with an expression of distaste.
Don’t tell him I said that!"
Steven Moffat, when asked whether or not we’ll ever get to see a childhood flashback of Mycroft.
(DenofGeek interview, January 2014 [x])