If you look at a site/product like Lumosity.com, you can compare your results to other age groups. If you accept that the tests on Lumosity do test particular brain functions/areas then its clear to see that from the millions of user that take part, the performance average does decline with age. It's actually quite dramatic to see that a 40 year old that scores in the 90% in a test category, fits in at 50% among the 20 year olds.