Because age is just a number... that tells us how incredibly old you are.
Joseph Robinette Biden Jr. was born on November 20th, 1942.
His teeth and hair were born quite a bit later and are likely immortal, but the point is: our President-elect is old. He's so old that "Robinette" probably seemed like a reasonable thing to put in the middle of your kid's name when he was born.
He's so old, in fact, that he's technically slightly older than the guy who is still president until January 20, and actively proving that old white guys are the milk-on-a-hot-day of politicians: a bad choice (also, they look and smell like spoiled dairy).