Christina Aguilera -- Good Genes or Good Docs?!

Christina Aguilera's is beautiful!

Here's an 18-year-old version of the New Year's Rockin' Eve headliner all dressed up in Los Angeles back in 1998 (left) and 20 years later ... The award-winning artist -- who turned 38-years-old just last week -- at an event in New York City earlier this year (right)!

What year is it?!

The question is ...