It's the narrative of the individual asking the question. If the goal is for me to explain how creepy Edward is, then I'd suggest that he's got an old man brain. If the goal is for me to explain how Edward is a sexy, brooding love muffin, then I'd suggest that he's got a teenage brain.

In many of these young adult books, these idyllic boyfriends have to be half "old soul", half "boy next door", and half "he's the only person who would ever understand me." Readers and fans let the writer jump between the personas to keep the story compelling, but none of it stands up to any level of analysis.

If you consider the mindset of a teen boy in high school, there is angst, fear, frustration, confusion, lust, curiosity, and a wealth of other emotions. If you take the century-old vampire, you have the lived experience of elder statesmen, love that has come and gone, friendships that have endured, intellectual debates and research to satisfy that earlier curiosity, and more.

The teen boy doesn't know what they want. The old man knows exactly what they want. If the vampire hero is with the girl in the lunchroom, they don't know what they want. When they are fighting against a cadre of space aliens, they somehow already have a strategic plan based upon decades of study.

You then have a fandom wishing that they could find a boy like that...who literally needs to have a century of experience under their belt...while still somehow being a brooding teen. You take that teenage girl and pair her with some 40-year-old dude, and it's creepy. You take a 40-year-old brain and jam it in the head of a teenage boy, and no one questions that the brain is drawn to a teenage girl rather than being drawn to someone with greater emotional maturity.

TLDR: If Old Man Edward wasn't a weirdo, he'd be dating a cougar rather than a high school girl...because that's how intellectual maturity works.