Just like fine wine, Irina Shayk is only getting better with age ... just take a look at her hottest shots in honor of her birthday!

The supermodel turned 39 years old Monday, though, it's safe to say she's defying time ... as she looks practically ageless as her 30s come to an end.

Take a peek at this little black bikini shot, where the Russian runway maven let her long, dark locks hang down as she zipped around on a 4-wheeler.

Irina is known for keeping things sizzling by stripping down to her birthday suit -- even on days where it isn't about her birth.

Case in point, Irina soaked up some sun practically nude while lying on one very lucky rock. It's clear Irina wasn't worried about any eagle-eyed photogs during this outing!!!

But hey, if we had a bangin' body like Irina's, we too would be stripping down whenever we got the chance.

For a closer look at Irina's hottest shots, be sure to check out our gallery above.

Happy birthday, Irina ... keep up the smokin' work!!!