Since her return was confirmed for BG3, I wanted to know how old Jaheira will be, as she is half-elf.
So I used... MATH! (I'm so ashamed)
Half-elves live for about 180 years, but if they live healthily and are lucky, they can live for more than 200 years.
Reach adulthood when they are 20, and are not considered old until they are at least 145 years old.
Their prime is very long.
Given the fact that Jaheira is a Druid and has a strong connection with nature (and probably a healthy diet), I believe that she could live without problems for more than 200 years.
We do not know her exact date of birth, but we do know that she was still a small child during the Year of the Bright Blade.
1347 DR = Year of the Bright Blade
1368 DR = BG1
1492 DR = BG3
124 years passed between the first and third games.
145 years passed since Year of the Bright Blade.
A small child should be about 5 years +/- few years.
Therefore, her age should be about 150 years.
Does it mean that when we meet her, she will be an old, unattractive fragile woman?
Not necessarily...
As I mentioned, Jaheira is a Druid (along with the monk, these are the two classes that care most about their bodies) which could help delay the old age by a decade or two.
I think in that case she could look like a woman in her 40s. She can still look very good.
BUT
A surprising number of humans from BG1 and BG2 are still alive.
Sarevok, Imoen, Valygar, Edwin...
Although some had some special power that helped delay their old age, potions to delay old age are not so unusual either (It has been confirmed that Valygar is using them).
In addition, Jaheira is a prominent member of Harper and Elminster is one of their friends. If she wanted to, she should definitely be able to access these potions.
Also, I suspect that the druids are the ones who prepare most of these potions.
RESULT
If we're lucky, it's still realistic for her to look like a 30-year-old woman.
But more likely, she will look like a 40-year-old woman.
If we are unlucky, they can made her 50-60 old woman.
