JELLY BABIES?

From the TV Tropes website:

Some form of Applied Phlebotinum results in one or more main characters regressing to an earlier age. It may be the result of a freak accident, a plot on the part of the bad guys to incapacitate our heroes, or done on purpose so the characters can relive their childhood (at least temporarily).

For some reason this plot is often used in Fan Fic, where it's called a De-Aging story.
When someone dies in his rejuvenated state, expect him to turn back old immediately.
It's common for characters to overdo it in stories involving this trope, often winding up as children, babies, or possibly even regressing out of existence entirely. This may or may not be permanent, but if the character in question was a bad guy, expect it to be permanent; well, at least, until they grow back up the old-fashioned way.

Here are the TV shows that have employed this concept:

'Star Trek: The Next Generation' - "Rascals"

'Kamen Rider Den-O'

'The Mighty Morphin Power Rangers'

'The Legend Of Dick And Dom'

'The Twilight Zone' - "Kick The Can" &

"A Short Drink From A Certain Fountain"

And even 'Doctor Who' dabbled in the concept of their regular characters turning into little kids within the story "Time Monster", although it didn't happen to the Doctor.

But since we've seen the Doctor aged by the Master, why can't the reverse be true......?

However the question arises - would a child Doctor look like the current incarnation of the Time Lord, or would he look more like a VERY young version of the First Doctor?

BCNU!