Shacking up with a young 20 something when you're creeping towards your grave would be fine for a month or two, but eventually that young 20 something is going to want to do young 20 something things, and all her young 20 something friends are going to be there, silently judging you, giving her pitying looks as you shuffle around next to her in your shabby old guy clothes, talking about old guy TV and old guy music they've never heard of.
Then there's your family and friends who'll be all supportive to your face while you explain how smart she is and how you both have so much in common; but once you've stuttered off on your new Harley with your hairplugs blowing in the wind, the same question will be asked over and over again - what the hell is going on there?
The only way to avoid any of that would be to only ever meet this young 20 something in secret. Never go anywhere together in public, especially not on your Harley. Never meet her friends and never ever introduce her to anyone you know.
That's impossible though because the very first and strongest old guy instinct you're going to have is to show her off to all your friends just so you can rub it in their faces and say, "Look at what I'm fucking! She's way better than that bitter old hag your tied to. Rock N Roll Man. Rock N Roll."