I think that Dragons can live as long as Alicorns, but they need a source of gems to eat, the more they grow, the greedier they get and vice-versa.
Basically, a dragon that expands needs more gems, becomes greedier, expands, becomes hungrier, starves.
My theory is that long-lived dragons would tend to be smaller, just like stars.
#281731Perpetual LurkerTue, 11th Sep '12 11:20:12 AM
It's kinda hard to say if X species lives as long as Y with no reference point. Here's what data we do have:
Dragons can take century-long "naps".
Dragon aging is potentially non-linear.
Alicorn aging appears to be linear, though it could potentially be affected directly by magic power accumulation (see: Luna's state before, while, and after becoming Nightmare Moon).
Alicorn aging appears to be at a normal rate during yearly years (See: Cadance), but slows as it progresses (see: Celestia's change in appearance over a thousand years).
Not much else to go on, so it's hard to make a direct comparison.
It's possible that between Filly Twi and Adult Twi that Cadence went up a tier in magic and that's why her appearance changed. So it's not necessary that they change due to time at all besides the longer they're alive the more magic they accumulate.
I'd say unlikely but possible; it basically depends on whether it could keep it's greed in check or not.
And that's assuming that Spike isn't a unique subspecies or something that ages differently...
I should really refine my headcanon on dragons. I have a basic idea of how I think it works (in way that explains how Spike ended up in Celestia's care), and how the puberty thing works, but I haven't really smoothed it out.