My dad's paid money to go to meet and greets with his favourite football team more than once. He's 60, they are all early/mid 20's. He gets all fanboyish over it.
I've paid money at nerd cons pre-covid to meet actors and actresses and get a photo.
I don't think theres anything necessarily wrong with grown adults going to meets with other grown adults, as long as behaviour etc is appropriate and least some effort is put into appearance and hygiene.
Saying that...
It still isn't appropriate in my opinion. They're young enough to be my own kids and in fact they're younger than my own daughter and really they are just children. One would wonder why middle aged men would be following them in the first place and I think you'll find it's more than just the music. Men do get more lecherous as they get older and I just feel it's creepy.
Yeah, I do think its creepy and weird for older adults (male or female, because sexuality cannot be assumed) to go to fanmeets with teenage/underage/non-adult celebs. BUT as weird as it is, again, if behaviour is appropriate, security is in place and everything is controlled, I don't know if it should be banned outright (and thats without going into the possibility of lots of lost sales). And I still think, in terms of actual danger to young idols, there are far bigger dangers behind closed doors. Look at allegations of Johnny's in Japan, or child talents literally anywhere.