Also, don't forget River Song recognized the Doctor and that from the look on his face it was"early days." Assuming she met him when in this 10th incarnation, perhaps she met him when he was older.
Personally, if I was going to end the 10th Doctor's era, I'd just have him regenerate from old age, dying the same way he did in his vision of his future as John Smith if FoB. I'd conclude the last special and then open the new series with a new Doctor, beginning with the aftermath of his regeneration the way Spearhead from Space and, for all intents and purposes, Time and the Rani opened. We know he's going to regenerate. Do we really need to see it? After all, we saw him almost regenerate, we've seen the Master regenerate, and we saw the 9th regenerate. To establish the "how" all he need do is look a mirror and say, "Ah, young again!" and leave it at that - the intention being that he had many more adventures as the 10th Doctor and died a ripe old age, and he and River Song lived happily ever after until she went to the Library. To estabish his age, he could tell somebody he was over a millenium now - similar to the way McCoy mentioned he was over 950 in T&R. That would get the Doctor's age "back on track," avoid the "900 years" thing, and leave room for Tennant to return at any point in the future w/o having to worry about him looking the part or not.
