He acts rather like the Woman appearing to Wilf in The End Of Time or The Moment appearing to the Hurt Doctor (they all like lighter coloured clothing than they normally wear) and yet we also know that Elizabeth made the Doctor the Curator and Tom used to play the Doctor.

We also know when two Doctors turn up from different periods the age gap cancels out in favour of the oldest version.

The Fifth Doctor aged in the presence of Tenth. The Second and Third aged in the presence of the Fifth.

So it could be the Fourth Doctor made old because the Eleventh (or is it the Twelfth or the Thirteenth) Doctor is in the same room and the oldest has forgotten their meeting.

Or maybe he is an older incarnation of the Doctor wearing his Fourth body again who really looks that old.

It's just an excuse to get Tom on screen really.