Donald Trump is one of the oldest sitting presidents in U.S. history ... and now he's completed another decade of life.

Trump is turning 80 today, and he's celebrating at the White House UFC Freedom 250 event, which is going down on the South Lawn.

POTUS is sure to be decked out in his birthday best for the occasion ... we're guessing we'll see him in his signature spray tan and a dramatic combover.

But the Prez wasn't always this old! Check out our gallery to see for yourself and take a stroll down memory lane.