Glenn certainly isn't getting any younger — he's 15 years the senior of Stori Musgraze, currently the oldest ever cosmonaut. But that's the whole point — NASA will be able to look at the same astronaut, then and now, to discover whether old age makes any difference in weightlessness.
Granted, Glenn is in such good shape that he's not your average old guy. But shooting him back into space is more than just a medical experiment. As Kluger says, "there's the romantic, emotional, adventerous value — everything that used to be the stock in trade of the space industry." Evidently, its glory days are here again.