I learned Go several years ago at the age of 20, and tracked how much time I spent on it. Comparing my progress with that of a certain strong amateur who started at age 14 and estimated how many games he played throughout his progression, I think I reached my current strength of 1-kyuu after roughly the same amount of practice that he had when he was 1-kyuu. This anecdatum of mine agrees with children's extra synaptic density leveling out around age 12. I would guess that it doesn't matter whether the writer was 40, 20, or 14; hypothetically, it might matter if the writer were 4 (or 7).