The paper by Dr. Salthouse is not without its critics.
The authors--Lars-GÃ¶ran Nilssonadef, Ola SternÃ¤ngad, Michael RÃ¶nnlundbg, and LarsNybergcdef--state in their abstract, “Salthouse claims that cognitive aging starts around 20 years of age. The basis for this claim is cross-sectional data. He dismisses longitudinal data, which typically show the cognitive decline to start much later, around 60 years of age.”
They add, "He states that longitudinal data cannot be trusted because they are flawed. There is a confounding between the effects of maturation and retest effects. We challenge Salthouse’s strong claim ….”
Additional information is needed to verify the claims by Salthouse.
To minimize the declines of mental aging, it might be prudent, in addition, to verifying these claims, to institute anti-aging programs earlier than is currently done.
Even though we think that aging starts in ones 60s, it might, indeed, start gradually over a lifetime and actually begin in your twenties or thirties. More research is needed, in any case.