Because she is chosen as a Sitter after reunification, it can be assumed that Farnah has been Aes Sedai for more than one hundred years and is, at least around 130 years old.
Strength and Abilities Edit
Being a Sitter in the Hall for a while means that Farnah is also a high ranking sister and a strong channeler by Aes Sedai standards. This means also that she is most probably above level 21(9) and that she has the strength needed to open alone a suitable gateway for Traveling.
"The Wheel of Time Companion" states at one point that Farnah replaced Talene Minly while at the same time stating that when Rina Hafden stepped down, it was Farnah who replaced her. However, it was Faiselle Darone who replaced Rina.
The Sitters try to hold a secret meeting without Egwene, discussing ways temper the Amyrlin's power when a declaration of war is called. Farnah voted for a proposal that would allow only Egwene to deal with the monarchy of the world, which includes Rand as he is King of Illian, while the Hall would take over prosecution of the war and deal with Gareth Bryne's army.
In that same meeting, Farnah voted in favor of the new rules and procedures in the Hall, which included temporarily replacing a Sitter when she is absent from the Hall and that no meeting of the Hall can commence without all Sitters and the Amyrlin Seat being present.