I'm 18, and occasionally I feel like I've lost something I had before. But that's a common feeling, I'm sure: I'll always want to be wiser and simultaneously younger. I doubt that has anything at all to do with my actual age or level of knowledge.
The feeling of losing things may very well stem from the inertia that builds up in life, along with your own growing awareness of your abilities. When I was 5 I thought I was a superhero: I was powerful and could crush anything. When I was 16, I didn't think that, but I thought that I was a powerful, fascinating individual around whom the world revolved. Now I've begun working out and taking care of myself, and I'm probably stronger now than I've ever been, but I don't have the same blind, false confidence I had before.
Is that losing it? I'll suspect that a year from now if I continue to work out with any level of dedication, I'll be stronger than I've ever been - not just in my mind, but in reality. Similarly, every year I become more aware of my limitations, but simultaneously those limitations lessen and disappear. I'm brighter now than I've ever been, and it's my intent never to enter a life scenario where I'm not allowed to continue to grow.
Inertia is the larger problem: both with body and mind and personality, the longer you don't use something the more it fades away. By your late 20s, you've left school, you've entered a more stable type of living, and there's every chance that you're in a duller, more monotonous setting - and worse, you're faced with the prospect of things never abruptly changing. That's deadly, and I'd suspect it contributes a lot to this attitude of people burning out.
(I come from a slightly unique perspective - the world of writing, where if you publish a book before you're 35 you're considered precocious. In all of literature, the greatest changes have come from people older than 30. I think it's because literature, unlike other forms of music, is forced to be subtle in order to work: it requires command of only a few skills, and there's no pressure to hurry with writing, and so as a result the field is full of people who can dedicate a decade to a single piece. The works that are remembered are the ones so rich with experience that they stand above even this field of work. Unlike, say, music, where not knowing anything means you have an advantage in your ability to create new works, literature very often requires at least an understanding of what's been done before, because without it the chances of your making something worth people's time is virtually nil.)