Jonah Keri has an interesting look, over at BaseballProspectus.com (subscription only), at the sudden development the last two years of two over-30 utilitymen (Mark Loretta and Melvin Mora) into major star-caliber players. He includes a chart of players who took major leaps forward after age 30, from a database going back to 1972. (Side note: here's an example of BP's insistence on using its own proprietary stats, in this case VORP, hampering its studies - they could have used Win Shares without any substantial change in accuracy and been able to run the study back another 100 years).

Anyway, I found the distribution of these leaps forward by older players over time interesting:

1973-78: 5 in 6 years. OK, we can discount that some due to the
paucity of pre-1972 data. Note that 3 are in 1973, when the DH was
introduced.

1979-92: 13 in 14 years, two of which were in the high-offense 1987
season.

1993-2004: 30 in 11 years.

Logical inference? Well, could just be a small sample size. But it
could be either that (1) physical fitness and simlar issues have created
greater opportunity for late-career advancement in the post-1993
environment, or (2) players can be affected unequally by an upswing in
offense, leading some guys to blossom more than others (consistent with the
1973 & 1987 results).