Should they age Matt SMith's character before they regenerate him? Though Christopher Eccleston declined to be a part of a "reunion"...would Matt SMith object to returning even years later to be in a multi-Doctor story? It seems to age him on screen first, so that gives him the freedom in the future to come back. What do you think
That is one way to go about it, but then where does Clara fit in? Did he wander off for a thousand years before picking her up for the Christmas special?
They probably wouldn't need to. It could be argued that in-universe he has aged already, as the only comments on his apparent age are (if I recall) from Amy in The Vampires of Venice and The Wedding of River Song. In the former she says "You look nine!" when he suggests that they pretend to be father and daughter, and in the latter she remarks that he seems to have aged since she last saw him. (This would be when the Doctor was "1103", seemingly about 2/3 of the way through his current incarnation.)
Artificial aging? no thanks. Although maybe they could do a scene in the Xmas special where the doctor finds his first grey hair implying that he's getting old and it is easier for him to let go at the end. But other than that, no, not really.
It could be a nice way to work around Smith shaving his head, but I doubt they'll do it. It was originally planed to have Tennant look older in The End of Time, but for whatever reason they decided against it.
I did not know that about the End of Time. This is interesting -- do you know why they decided against it?
Previous multi Doctor specials have seen actors looking older. It's not a big deal. There was a throwaway line in Time Crash, something about temporal potential making Five look older, and he'd return to normal back in his own time.
I would guess any number of reasons. They save time and money by not putting age make-up on Tennant, even if it is just something simple like making his hair gray. They risk it being too much of a distraction if they don't get it just right. The desire to see the Doctor in his traditional look the last time we see him, which I believe is the reason for why his cuts, bruises and scars instantly disappear after his radiation exposure begins the regeneration process.
There's also the fact that even if you age an actor there's no guarentee that's how he'll actually look when he's older anyway!