Justin Theroux -- Good Genes or Good Docs?!

Justin Theroux's good looks are pretty doggone good!

Here is a 28 year old version of the "Mulholland Drive" star rocking some stellar specs at a party for "Frogs for Snakes" in New York City back in 1999 (left).

And, now, 20 years later ... the awesome actor -- who is celebrating his 48th birthday this weekend -- was photographed showing off some scruff earlier this year in NYC (right).

Theroux has been tapped to voice the character Tramp in the live-action remake of Disney's puppy love story "Lady and the Tramp."

Nothing ruff about these good looks!

The question is ...