I feel the opposite. I'm 61 and I feel like I understand ideas more quickly than I ever did before, so much so that I'm surprised at how shallowly I thought about some things in the past.

While there is definitely something to the plasticity of young brains, for example in language acquisition, or the fact that the Fields Medal eligibility ages out at 40, I believe it's not a linear thing and not a one-way thing.