I think the best asnwer is, we don't know.
In Teenage Mutant Leela Hurdles, the age-ometer says he's 161. But, after the whirlpool scene, another age-ometer takes another reading. This is probably not exact, but the dialog goes something like:
<Age-ometer is used and produces a reading that the audience doesn't see>
Leela: Are you the right age again?
Professor: Even older!
<body cracks under stress of holding up his own arms>
But since we don't know the reading, we don't know his age.
His brithdate didn't change, though, so his age and his age-reading might be different. But wouldn't an age-ometer worth its salt read actual age? Meaning, his birthday did actually change? (The age-ometer knows your birthday, right, Amy?)
