Guardians may age, as proven by Drifter, but it takes an ungodly amount of time. The reason Drifter looks older is because when he dropped the Haul into the singularity on Kepler, his physical person underwent a massive amount of Time Dilation, aging him up hundreds of centuries in a rather small amount of time. Edit: mind you, Drifter has looked virtually the same prior to this event from when he was first risen. He is one of the oldest Lightbearers we know, meaning that over the course of centuries, Lightbearers don’t age much. Edit 2: even Ex Guardians, being characters who’ve lost their Ghosts age much, much slower than the average human. And mind you, the average human can live to be about 300 because of the Traveler. Characters like Eris Morn and Shin Malphur are examples of this, although Eris might be an exception as she is a technically a God, albeit a lesser one now since she gave up all that power in Season of the Witch. Malphur might also be an exception as he has an abnormal amount of light despite being an Ex Lightbearer. So much so, that he is still able to manifest a Golden Gun in spite of the fact that he doesn’t have a Ghost and is now mortal.