Paul Rudd -- Good Genes or Good Docs?

We're clueless as to how Paul Rudd's good looks just keep getting better!

Here's a 28-year-old version of the "Knocked Up" star at a movie premiere back in 1997 (left) and 21 years later ... the "Ant-Man" actor -- who just celebrated his 49th birthday -- at a benefit in New York City just last month (right).

'I Love You, Man.'

The question is ...