Nelly -- Good Genes Or Good Docs?!

Nelly's looks over the years will make you "take off all your clothes!"

Here is a 27-year-old version of the Grammy-winning star backstage -- swagged and styled in diamonds -- during Z100's Jingle Ball at Madison Square Garden back in 2002 (left). This was just one year before he took home his first Gramophone trophy.

And, 21 years later the 48-year-old father recently shone bright with a head full of diamonds at the 'Clay Day Celebration for OG Clay Evans' event in Atlanta, Georgia earlier this year (right).

Nelly is set to perform this weekend at the "Life Is Beautiful" music festival in Las Vegas ... "Hot In Herre!"

The question is ...