Katherine Heigl -- Good Genes or Good Docs?

The truth of Katherine Heigl's looks are anything but ugly!

Here's a 15-year-old version of the "Knocked Up" leading lady at a movie premiere in New York back in 1994 (left) and 24 years later ... the "Grey's Anatomy" actress -- who just celebrated her 40th birthday Saturday -- at a press conference this summer (right).

Age suits her well!

The question is ...