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 ...