As kids, you can get away with having the saddle too low because kids never ride lots of mileage continuously at high speeds. But once you starting doing more than 10-miles at a time, the repetitive motion becomes more strenuous on your knees.
There are various fit ideologies out there, and they're all pretty close to each other. One method prescribes putting your heel on the pedal at the bottom of the stroke. Your leg should be straight. This is actually the minimum height to have and all the other fit techniques will actually have your seat even higher. Track racers tend to have their seats about 1/2" higher than this even.