David Brooks provides a nice summary of "new" growth theory and the macroeconomics of innovation in his column, drawing from Paul Romer's work as well as several others. Romer's quote on the trend in the average age of NIH grantees, now 50 instead of 35 in earlier periods, was interesting.
A 2006 article in Science by the NIH Directory Elias Zerhouni implicitly pointed out another trend, in addition to the rising age at which scientists receive a first independent NIH award. The average age at starting a tenure-track faculty position was 38 in the data he examined, surely higher than in earlier periods. Lengthier training periods might explain part of the increase in average grantee age.