I have read [no citation available :-( ] the actual crime rate is somewhat correlated with the number of young men -- say, aged 13-30 -- in the population. This would explain an increase in the 1960s followed by a decrease in the 1990s and beyond.
That said, age does play a significant role in crime rates. Our population is aging, and that's probably the primary cause of the decreasing crime rate.
The Gen-Y baby-boom is not a larger group than the original Baby-Boomers. However, there is definitely a noticeable Gen-y bump. I'm guessing that the most common ages for criminals are in the dip of that curve, maybe 20-24, which means the downward trend should soon begin to reverse if this correlation holds true.
edit: just realized that chart is from 2000; you're absolutely right that a second spike is missing.
I'm joking, but the point is serious -- if youth are the main demographic to commit crimes, and our youth now spend a lot more time indoors/online... wouldn't it make sense that the internet is actually responsible for keeping hoodlums off the streets?
The second is discussed in Freakinomics and the first was discussed in a follow-up article by the same authors. I've listed them in the order I did because my memory says that reductions in lead are believed to have had a greater impact on crime rates than increased abortions.