From my perspective, Jaheira already got her romance (which if you don't mind the widow thing, and can avoid the bugs, is actually pretty good and has more content than Aerie, Viconia, or Anomen in BG2). I don't particularly see a reason for her to get another one.
Edit: also, Jaheira is a half-elf, not a full elf, so she does not have an elven lifespan. Her longevity is probably due to being a druid, I'd expect. I always placed her in her late 20's/early 30's in BG2, and it has been over 100 years since then, and half elf lifespan is only like 120, if I recall correctly.
Depends on source material I think. Tanis Half-Elven was 100 (or very close) and he was just beginning middle age. I'd have to look it up for Faerun, but I think its more than 120 years.
