18, 262 days.
Had he been born on Jupiter, he'd be a smidge over 4 years old (but then, he'd be more likely to live in Europa's underground moon-spanning ocean, with the other alien squiddies). Mercury? 207 years old. Venus? 81. Pluto? Just 2 and a half months old.
But here on the green, green hills of Earth, PZ Myers of Pharyngula is 50 today.
50! That may be a tiny fraction of the lifespans of Moses, Abraham, and Methuselah -- there were giants in those days, after all -- but it's still rather impressive for a guy who seems to live for ticking off creationists. As PZ himself would tell you, it's advances in medicine due to evolutionary (gasp!) biology that allow so many of us to live to our three score and ten, and well beyond.
Let's hope that PZ beats that and then some. We could use more fiery rhetoric like his.