Kelly Slater -- LANDS 540 TWIST ... At 42-Years-Old

Age ain't nothin' but a number to Kelly Slater ... 'cause the 42-year-old surf legend just hit a 540 TWIST during a session in Portugal.

It's an insane athletic feat for anyone at ANY age ... but the fact Slater was able to pull this off in his 40s is especially impressive.

There's an argument whether the jump is a 540 or a 720 (looks like a 540 to us) ... but Surfing Magazine is calling it a 540, so we're gonna go with that.

Sickness.

While we're on the topic, here's what a 720 DUNK looks like ...