The protagonists of the Sword of Truth novels by Terry Goodkind are Richard and Kahlan. How old are they throughout the course of the series, especially during the events of the first book, Wizards First Rule?
I've only read the first book, so not sure how much time passes in the whole series, but from what I can tell, they are both mid-twenties in Wizard's First Rule.
In Stone of Tears, when Kahlan enters the "children's army" camp, this passage is given:
"How many children do you have along with you, Captain Ryan?"
He stiffened a little. "Mother Confessor, we may be younger than you, although not by much, and you may not think highly of us, but we are soldiers. Good soldiers."
"Good soldiers." She was hardly able to keep herself from screaming at him. "If you are such good soldiers, why was I able to walk, unnoticed, through your line of sentries?" His face reddened and he made a visible effort to remain silent. "And is ther a one of these good soldiers, including you three, that is beyond eighteen?" He pressed his lips tighter and shook his head. "The I repeat, how many children do you have along with you?"
So, seems like a) someone is considered adult around 18 and b) Kahlan isn't much beyond that.
On the first page of Wizard's First Rule, we are told Richard moved out of his "father's" house "about five years before". Likely shortly after reaching adulthood. Assuming this, he's mid-twenties, Just like Kahlan.
This is supported by the passage when Richard first sees Kahlan:
Richard came partway to his feet. Waves of heat rolled from his body. Bent over at the waist with his hands braced on his knees, he took a few deep breaths before straightening to his full height.
His eyes fell on the young woman as she came around the bend in front of him. His breath caught for an instant. Her brown hair was full, lush, and long, complementing the contours of her body. She was tall, almost as tall as he, and about the same age.
Later books may have passages that expand or contradict this, though.