Compared to the protagonist, she was already quite a bit older in BG2. So if it wasn't an issue back then, it shouldn't be one now? Half-elves live quite long after all.
You can make your character older too - a golden age romance would be kind of sweet tbh.
Yeah. It could be a great romance.
Sadly, I very much fear that Larian decided against letting us romance Jaheira.
So we'll be able to romance a buff Elf in hairy bear form, but apparently not a mature Half-Elf woman ...