A UCLA study published in the Neurobiology of Aging found that age related decreases in myelin correlate to decreased motor function after the age of 39. The researchers suggest that sensory and cognitive processing speeds are also effected by the loss of myelin.
That would be problematic for scientists. The age at which U.S. researchers get their first NIH grant increased from 34.3 in 1970 to 41.7 in 2004, according to a recent paper on arxiv.org by Yves Gingras et al. The authors studied 13,680 university science professors and showed that productivity rises between 28-40, rises more slowly between 41-50 and decreases until 50-55 years old. Although it's a measure with limited value, the authors counted "productivity" as published papers. For multi-author papers the study credited one paper per listed author. In short, the study found that scientists still produce papers up through retirement, publish in well respected journals, and are cited more frequently.
Neither study is earth shattering, but there's value to aging. Motor processing does slow down, but other studies have shown that with some motor function, movement accuracy improves, compensating for decreased speed. For many reasons, lab dynamics, prestige, networking, etc., older scientists may not publish more, but their quality of production (by measures by which people judge, anyway) may increase.
