She was 16 in the flashbacks, soon-to-be 17 if my memory serves me.
And she stayed like in a time capsule during those 100 years holding Ganon back, just like Link after waking up in the Shrine of Resurrection. They were both still teens at the end of the game!
Although in Link's case, he must have been around 35 by the time I completed the game considering that it took me 400 hours to 100% it plus the DLC