I fear that it's really not much a matter of belief, those things have been studied for half a century. Cognitive biases do exist. The brain does change with age.
You, my friend, certainly aren't representative of the way most people think, I can tell by your posts. Our brains aren't all the same and maybe also you have decided not to lose your sense of wonder, which helps a lot. I know I don't want to become like that.
Nevertheless, look at most people over 60, take a little survey: How many still listen to new music? How many enjoy recent music? How many will you find who won't tell you that music was better before and that they have no interest in current music? There has to be a reason why those people who are 70 today got stuck in 1980. And again, maybe your grandma isn't like this. But most grandmas are. Take the survey if you won't take my word for it.
I know they've been proved. And that's the point: they should not be considered as normal, inevitable, but rather social distortions that can be fixed by social readjustments. Once man realizes one of its flaws, he usually acts upon it, hence the shrinks. Pyschiatrist is a profession born of mankind realizing her biases bring damage to themselves and their relationships. They're just coaches for the mind digging, which is ultimately a lonely process.
In fact they're both social and biological distortions but both can be corrected: we know know the brain has great plasticity, for example the beneficial effects of meditation have been scientifically demonstrated. However what we lack is an awareness of those things. I still can't believe psychology and neuroscience aren't taught to every secondary and high school child in developped countries!! My father, at 79, still has no clue what a cognitive bias is (and no interest in learning what it is), and he assesses the information he gets about politics and everything else without this awareness: the result, unsurprisingly, is quite horrific intellectually speaking.
The only reason the twenties are polaroided in people's brain comes from society giving each decade very specific roles. We know the thirties are still considered the decade when you're supposed to start a family. Likewise, curiosity is supposed to end with school (in the hope it was ever sparked). But where I come from, we don't let society, etc.
Again, we know there are also biological factors involved, but yes, clearly youth is idealized in modern societies, and people are expected to do/experience things differently according to each age category..
SPeaking about biases: how come young children spontaneously help? How come young children naturally collaborate when left to their own devices ? Three years old in control? I'd feel much safer. So biases distort, they are flaws. It's okay to put the diagnosis down, but only capitalism will have you believe your pregiven role is something you should be okay with. It kills the dreaming of possiblities.
Honestly, I have quite a different experience with 3 years old: I've worked in a Kindergarten, teaching ESL there, and I've seen some (not all, but quite a few) of those babies willing and able to crack each other's skull for a toy at age 2/3. I've seen some of them display incredible violence, it was quite shocking! It's only when they began to integrate the processes of socialization and manners, after being taught the concepts of sharing and helping each other out, at age 3 to 4, that they began to cool down. After this work experience I could never take A Place In Heaven seriously. 3 years old in control? Not in my world!