Jack Black -- Good Genes or Good Docs?

Jack Black has some rockin' good looks!

Here's a 31-year-old version of 'School of Rock' star at a movie premiere in Hollywood back in 2000 (left) and 18 years later ... the now 49-year-old 'Tenacious D' rocker -- and father of two -- was spotted showing off his scruff in Los Angeles earlier this month (right).

This is just a tribute!

The question is ...