I think it's not about the physical state of your brain, but more about your body, your life and your consciousness. As you get older you'll notice that you are not feeling that well anymore, daily routines take more time to complete, you just can't take your body for granted anymore and forced to take care of it, more and more the older you become. Also as time passes there is more chances that some shit will happen that'll kill your drive to do things, or take away your will to live (and will to learn things with it, obviously), and some (most?) can't get out of this till the end of their lives. Also, in most cases, you can't really rely on your parents support anymore as you get older, and instead of learning most people are forced to get shitjobs so they can pay for food, rent and whatnot, and on days off all you can usually do is just try and relax, because with that kind of daily routine chances are you won't be able to learn effectively even if you try. I don't believe that brain gets significantly weaker until about 60s or even later, if you are more or less healthy. I may be wrong though.
