There really isn't a strong case either way on the matter of mid-life cognitive decline. The peak is "somewhere" between 17 and 70, but the plateau is so flat that it's hard to pinpoint it to a specific age and it seems to depend a lot on the individual.
We still don't know when we peak. I tend to take that as a positive.
Where there is exponential worsening is in physical health: risk of death (also, risk of cancer) go up at about 9% per year, or a doubling each 8 years-- that's the Gompertz model, and it holds well from about 20 to 85. For mental decline, though, evidence for general decline in healthy people is pretty thin.
We still don't know when we peak. I tend to take that as a positive.
Where there is exponential worsening is in physical health: risk of death (also, risk of cancer) go up at about 9% per year, or a doubling each 8 years-- that's the Gompertz model, and it holds well from about 20 to 85. For mental decline, though, evidence for general decline in healthy people is pretty thin.