Dear Tom/The World of Who
My own opinion - and it is only an opinion - is that Moffat has done a genius thing. In the ""Day of the Doctor", we are certainly left with the prospect that Tom is still the Doctor, albeit slightly older but somehow a future body of the past. Anyhow, "Time of the Doctor" shows that the Doctor can age within his own timestream. Of course, all Doctors' timestreams finish when they regenerate. These are the current rules but anything is possible in Who! I don't see the fuss over the new regeneration cycle - it was offered to The Master in "The Five Doctors". Gallifrey still exists, so it all seems logical that the Timelords intervened, from wherever, through the Crack in Time (created by 12 Drs averting the Time war and hiding Gallifrey) .......which it seems that was caused by the Time War/Doctor intervention in the first place! For Tom still to be our Doctor, it requires that he left via the Watcher - into a parallel universe, as Jon's Doctor did briefly. But Tom's Doctor was stuck, getting older, and suddenly got the chance to nip back through the crack in time (caused by the Timelord kerfuffle) to talk to one of his previous selves. It all makes perfect sense. But WHO KNOWS?
Tom - I am just one of millions who had to wipe away the odd tear when your voice came booming through in that episode.....and then you appeared. A fitting end to a wonderfully written script.
PS Still haven't got over the ciggie problem!!!