I've run into a few women of my own age or not much younger who apparently haven't looked in the mirror lately.
They act like cute college to twenty-five year olds (and they might have been, fifteen or twenty years ago) but they are now old enough to have their own kids graduating from high school and going to college (if they had kids) and at 40+ - the mannerisms, vocal tricks, poses, and facial expressions - not to mention the choice of clothes & jewelry - can be jarring. Some of them are even older than 45...which is where it really looks odd (black hair dye at 50 might make you look even older than leaving a few lighter "highlights" here & there).
There was an expression in the nineteenth and eighteenth centuries about "Mutton Dressed As Lamb" (for today - Baby Boomer dressed as Generation Y would give pretty much the same information).
Whether the person is male or female, acting like you are at least a generation younger than you really are only works on stage, with lots of makeup, and everyone KNOWING that the actor is playing a part. In person, complimenting/hitting on someone who is "not interested"? It goes from "slightly humorous" to "feels vaguely creepy" to "urk, dirty old man/woman alert" depending on how they are approaching people around them. If they think that it's still the 1960s, they might be facing medical issues....but if they know it's the 2000 & teens, then.....it isn't funny.
Talking with people is one thing - trying to flirt with them or get treated as "one of their peer group" is another.
It might be fun to be told that you look no older than you did in high school at your 25th reunion, but if you believe the person saying it, then in real life they are trying to sell you plastic surgery, expensive moisturizers, or something. In reel life, they might have a wooden stake in their back pocket & they are working with Buffy, the Vampire Slayer.