Maybe they feel safer by you that by a young man!
Maybe they never had kids/grandkids.
Maybe you look like someone they knew.
Maybe they made the mistake of thinking they're sitting my a sensitive, decent human being who is aware of the fact that they too will soon be an "old" person who gets dissed and ignored by future young people.

FFS this is what developing a youth culture mania has resulted in.

C'mon, give us old farts a cheap thrill. When you're old and fat you're going to plop your flabby ass next to a young fireman

You mind your own business. Don't like it - move. It's a free country last time I checked.
Fuck off

... they're senile enough that your not-fooling-anyone verbal cues don't garner immediate obedience? Most fun is walking around a mall, ignoring female nonverbal cues. They get so angry!

There is a free seat next to you. If you don't like me sitting in empty seats on public transit you should buy yourself a car or a bike and shut the fuck up.

When a guy turns 20, he wants to be with a 20 year old. When he turns 40, he wants a 20 year old. When he turns 60......yep, still a 20 year old. I think this is widely known. Heck, Mick Jagger just fathered a child with a woman young enough to be his grand-daughter. So, maybe grandpa likes to think he still has a shot?

Maybe he's gay and didn't even notice you. That's harsh but please stop thinking​ the world revolves around you. If you find it so creepy, be just as weird and find a different seat. - from a 24 year old woman.

Should take your medication and stop listening to the voices in your head.
They can sit where they damn well please.

I just think it's in all your mind

And I can't help but think you're a little self-absorbed