Here is a 21-year-old version of the "John Tucker Must Die" star showing off his smooth looks at the Teen Choice Awards back in 2000 (left).

And, 19 years later ... the stubbly star was just spotted in Los Angeles at an event for the Hallmark Channel's "Countdown To Christmas" (right).

Metcalfe recently starred in one of the network's Christmas movies, "Christmas Under the Stars" ... and it seems like it's this fella's season to celebrate more than just the holidays, because Metcalfe is spending the weekend celebrating his 40th birthday!

Everything is merry with looks like these!

The question is ...