It's a side effect of her using her Gate Power. They don't find this out at anytime during the Origional Gatekeepers Anime. But in Gatekeepers 21 we find out there is a side effect to using Gate Power.
To be born special with the ability to summon a gate comes with a heavy price. If at an early stage a Keeper is not aware of his ability the gate will absorb his happiness in the future.
That is why almost all GateKeepers have some kind of sadness in their past. Note on the backgrounds of Shun (father died), Reiko (broken family), Kaoru (her ability was not recognized and was even disqualified for achieving impossible record on track and field), Fei (family was taken away from her by the government), Megumi (her insecurities towards Ruriko or someone better than her), Reiji (father got broke and dies while his uncle takes away their belongings), and as for Ruriko - her heartaches towards Shun and also her insecurities in life.
If a GateKeeper overuses his/her power, the gate will consume his/her physical being or time.
In Yukino's Case her gate has begun to consume her time which is why she doesn't age.
Overuse of your gate can and will kill you. In Gatekeepers 21 we find out that in Yukino's case it has come to the point that if she uses her powers only a few times more then her gate will consume her life force and kill her.