Hello
Time and in effect AGE are altered by gravity and the increase and reduction of its effect on humans, as they travel through space.
Hence, any biological computation of a characters age, would have to take into account the time, distance and degree of reduced or increased gravitional effects on their cell structure.
The base point would have to be the cell aging rate on their birth planet, or system, and then adjusted for variances when they are on a planet. Then you would futher need to adjust for the time traveled in space, and the distance form any major gravational body, which is then adjusted again witht he distance form that body, and the speed at which they are traveling. ie - time distory 1, etc
Have fun !!
tony