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I think it has to do with people not being able to feel a significant change in mental fortitude after the age of 20 or 30.
I have had conversations with older people. Alot of them agree they don't feel "older" until they look into the mirror and see a significant change compared to their youthful body. And thanks to, pictures, you can show them the difference.
And time is this stubborn illusion that keeps people in this weird mental state. Everyone will agree their body is feeling older and sluggish. But they feel that their mind hasn't dwindled away.