XTOL is supposed to last for 6 months, according to Kodak. Their estimates are usually on the conservative side, so if you mixed it "before Christmas" (December 2008), then it's only four months old, and should theoretically be OK, if it was stored properly. That last is very important, though; if you put all your XTOL in one big bottle that you've been using, then there's presumably a lot of air in the bottle by now, which will speed the developer's deterioration. There could also be unknown factors doing the same (contaminants in the water, temperature issues, etc.).
All in all, I'd advise caution. If you absolutely must develop this film before you can get ahold of more XTOL, you might consider mixing up some caffeinol. That's got its risks too, though, at least if you've never used it with the type of film you want to develop.