Haven't read yet but is it rose you're talking about
Nah. Rose is in her mid twenties. If I remember correctly she mentioned that she's 25(?)
She started training when she was 12 y/o, left her home with 15, somewhere between that and the age of 20 she joined the knights and was raised to battalion commander at the age of 20. Then she lost her squad around the same age, left the knights and formed the rescue squad. When she was 22 the war with the demons started anew and three years later she met Usato and the heroes.
So she has eternal youth due to her healing magic??? Cause she's definitely old from that story.