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When Amy hit on the doctor (at the end of "Flesh and Stone") he said
I'm 907, and look at me. I don't get older, I just change. You get older, I don't.
Yet on Trenzalore, we see him age very considerably
Was he lying, does he age very slowly or is he expected to die/regenerate soon?
Yes, in a sense The Doctor was lying to Amy, though only in the most strictly technical sense. He was mostly trying to emphasize to Amy that he was not human, and that there was no way a long-term relationship between the two of them would work. Had the two of them stayed together and continued to travel in the TARDIS together, she would have gotten very old and died long before he showed any signs of aging.
First of all, out-of-universe, the obvious answer is that Moffet hadn't yet decided that there was a "missing" Doctor, so Eleven's dialog in the first few seasons would have assumed there was at least one more regeneration coming. This is clearly what he meant by "I change" in the scene you mention.
In-universe, lets assume Eleven already knew he was really number 13, and did not expect to regenerate. In this context, his comment makes no sense to us, because we've seen all 13 of his life spans, and seen him get very old even before Time of the Doctor. In particular the First Doctor actually died of old age and regenerated into Two, so clearly, he ages.
But The Doctor was't necessarily trying to tell Amy what to expect if the two of them stayed together for an extended period of time. Rather, he was trying to impress upon her the huge difference between his species and hers. In this context, what he was saying was a description of how Time Lord's biology works. He was also likely drawing on his past experiences of seeing people he loved grow old and die. The episode "School Reunion" features a similar scene between Ten and Sarah Jane Smith, who was considerably older than when the Doctor had last seen her, while The Doctor (a different incarnation) still looked very young. This was the kind of image Eleven was trying to portray to Amy.
In my opinion, the most important part of that speech was the last bit, where Eleven really emphasizes his point:
You get older, I don't, and this can't ever work.
He was just generalizing. His body can age (and can appear to be different ages across incarnations), but psychologically, he can only be described as "really, really old," so to him, it doesn't really feel like he ages.
