Where do you think all that 'humpin-the-guitar' stuff came from?
Tryin' to get your rocks off on stage, that's what. Is it normal? Yes, in the fact that it's a normal heterosexual thought, even if you are old & grey and the girl's really young,... in fact that's probably more normal even still. Just be glad your bone's not gone limp, that you've not lost all desire, & you're not gay.
Gettin' boners watchin' young chicks strutting their stuff? Sure, it's normal. I'm sure they'd be flattered if you showed them, they'd probably give you more tease. We're not talking about your daughter, or anything like that, are we? Anyway, have fun with it,... uh,... I mean,... not literally,... not onstage anyway, haha, but just have fun with it.