I think there are two things at work: (1) how fast you feel time passes, and (2) how long ago a point in the past feels like. If there is not much going on in your daily life, you feel time passes slowly. But paradoxically, you also feel not much time has passed. If the same things happen every day of your life, the lack of variety makes you feel you haven't lived very long. So it's important for anyone at any age, retired or not, to seek new adventures every so often. A working person in repetitive daily working routines are in need of that, and so do retirees with all the time in the world.