It’s because you do less memorable things, so there are less milestones to judge the time by. When you’re young, you don’t have to try so hard as many things are new and exciting. As you get older, you get a career, fall into autopilot mode, and maybe just take a vacation once in a while. Your options are to either figure out how to free yourself of this system so you can spend your time exactly how you want, or to rediscover the wonder and reward in small, seemingly everyday things.