He’s the Eighth Doctor.
He lived out his life to the point that he became older, because he’d lived through the many years of the Time War. And he was the one to end it. He’s the one who wasn’t the Doctor anymore.
And at the end of his life, he died, becoming the stubborn and serious Ninth incarnation.
It’s one of the few parts of the Doctor’s life we’ve never seen. And it’s the one part of which he’s most ashamed.