My 2E son could read digital clocks at three. He learned how to read an analog clock before he turned five--around the same time he started counting the amount of change I gave him. Once he was able to count by 5's easily, reading the analog clock was easy.

But there were some analog clocks and watches that he was not able to read. It took me a while to figure out that it was because there was not enough difference in the length of the clock hands.