She’s much older, yes. But there’s a difference between falling for someone who is an adult by the standards of their own species, and also looks like an adult by the standard of your species (are elves and humans actually separate species in Middle Earth? IDK), and falling for someone who looks like a minor and is considered a minor by his species. She could still have been the first if she fell for him if he was 25, and that’s still way different from falling for him when he was, say, 50 (just because a 50yo person will generally be a lot more mentally mature than they were at 25yo).
Dude’s like 80yo, there have definitely been women who fell for him before. Unless Arwen really likes 'em young and got there first.
Isn’t she 2500 years older than him ?
She’s much older, yes. But there’s a difference between falling for someone who is an adult by the standards of their own species, and also looks like an adult by the standard of your species (are elves and humans actually separate species in Middle Earth? IDK), and falling for someone who looks like a minor and is considered a minor by his species. She could still have been the first if she fell for him if he was 25, and that’s still way different from falling for him when he was, say, 50 (just because a 50yo person will generally be a lot more mentally mature than they were at 25yo).