I know my DS8 really had it mastered by 4 1/2 - when he started his final year of preschool. At that point, he wanted to wear an analog watch and could tell time to the minute and do things like tell you what time it would be in 20 minutes or what time it had been 1/2 hour ago. I remember the watch because the preschool teacher thought it was pretty unusual. I am not exactly sure on when he started to understand it though.