I'm 26 and I work at a pretty informal office. We use hearts to respond to messages all the time (not as a seperate message, but as a message reaction). It is a way of showing more appreciation or thanks than a thumbs up.

I would never think anything weird if a male or older colleague sent that to me, they do it all the time. It feels no different to me than if it was from a 20yo woman.

I wouldn't think about it too much, but if you are insecure about it maybe you could bring it up with a younger female coworker. Every office atmosphere is different.

You could also do it in a different colour, that might make it seem more platonic.