As you can see, on average guards do disappear sooner than players at other positions, starting at age 32. The conventional wisdom is that smaller players who rely on their speed/quickness tend to lose that ability (and therefore their viability as NBA players) in their early 30s; bigger players, of course, lose it at that point too, but it doesn’t matter as much for them because they were never very fast to begin with. Based on these results, I’d say the conventional wisdom has been confirmed — guards do appear to “lose it” sooner than big men, starting in their early thirties. That makes the recent re-signings of aging PGs Jason Kidd and Mike Bibby particularly risky, and the buyer should certainly beware when it comes to Allen Iverson, who could end up being the poster child for this phenomenon.There's lots of interesting* data and charts for those inclined.
*There's a 59% chance that Jeff Teague peaks between his third and sixth year assuming he plays at least 2000 minutes in an NBA season.