Robert Downey Jr. GOOD GENES OR GOOD DOCS?

4/4/2015 1:02 AM PDT

Robert Downey Jr. -- Good Genes Or Good Docs?!

Robert Downey Jr.'s good looks have been resurrected!

Here's a 21-year-old version of the "Back To School" stud leaving a movie premiere in New York City back in 1986 (left) and 29 years later ... the "Iron Man" -- who turned the big 5-0 this week -- at an awards show earlier this year (right).

It's a mystery dear Watson!

The question is...