The Development Modifier influences how fast or slow a player reaches his peak. The Aging Modifier influences how fast or slow a player drops off. It can be fairly complex to think about on a skill-by-skill basis, and unfortunately we get only a macro "pitcher" or "hitter" toggle that influences all ratings for the player type.
I selected to develop players more slowly because I was seeing both pitchers and hitters peak far too soon (ages 23-25 rather than 27-29), as experienced in game, and "proven" via my Tango comparison.
I selected to age players more quickly because I was seening them hold on too long, and because the Tango study showed they needed it to model major league careers better. Yes, some of the issue with Ks falling too soon are aggravated because I increased aging. But if you look at the charts in my report, you'll see this is due to the fundamental curve
OOTP is using. It drops off too fast after age 37, meaning you can either set it so that you mid-range is too high, but old pitchers hold on properly, or you can set it so that the aging curve is "right" until age 37, but falls off after that. Given that there are more pitchers in the mid-range ages, I chose to make that part right at the expense of the older pitchers.
Ultimately, given the game's approach to development, each setting is a trade-off of some sort.