I don't think becoming mentally fixed as you age is inevitable, though I think I'm right in saying that neuroplasticity decreases markedly at some time in your mid twenties, so it becomes more of a struggle—plus, of course, you begin to feel less physically and mentally energetic, you have less friends, you're less outgoing, etc. (Being in touch with young people is important, too, because they'll take you to task on your jaundiced view of things sometimes.)
(I think I pretty much always looked down on younger people even when I was young myself, I don't think that's something that only develops in your 30s/40s, you always look at the young with fear, envy and derision—at least on a kneejerk level.)
According to a book wot I read, psychedelics are good for shaking things up in your calcified mind, temporarily removing obstacles that stop you forming new neural pathways in your brain, etc. There's certainly a way in which they open up your senses and mind to new ways of seeing things, and to seeing things at all—because, as you say, the more you see something the more you take it for granted.
I'm a bit obsessed with the loss of mental powers that I think is happening to me even as I type this twaddle. I feel like nothing I learn is retained. My vocabulary is withering away. I get the odd day like on Sunday when suddenly I feel mentally alert and able to rub two sticks together to get some sparks going but those are rare.
Youth is wasted on the young in a sense because when you're young you don't know what it's like not to be young. But that naivety is essential to the glory/folly of youth. If you started getting physically younger in your 40s, say, you would regain those learning powers, the libido, the ability to go on giant drug binges without feeling like killing yourself after, etc. And perhaps the excitement of that would be enough to regain the optimism? (Esp. if getting younger again meant getting further away from death.) But I think it wouldn't be the same.