I think there are a number of reasons why 30 marks a turning point for adults' lives.
By that age, most people have been a part of the workforce for about ten years, and they may be discovering that they're not as successful as they'd dreamed they'd be. They're no longer feeling excited about joining the world of "grownups" like they did as teens, and the excitement of their new career has worn off as their responsibilities (and debts) have grown. It's a time when they may feel stagnant and wanting change, but haven't figured out how to make that happen.
An interesting sidenote is that in astrology, the planet Saturn takes about 28 years to complete its orbit around the sun. That means that it's near the same position when you're 30 as it was when you were born, which astrologers would say produces a sobering effect that lasts for several years and places a lot of focus on responsibilities, restrictions, and duty.