I have good sleepers. DD is 6 1/2 and she still NEEDS a nap every 9 days or so, especially if she has been up late (past 8:00). No amount of getting to sleep early will help her. In fact, she can be so overtired that she can't get herself to sleep at night and the problem just gets worse. She is unbearable and totally irrational

without the nap. Last week we had a couple of late nights in a row and she went to bed between 8:30 and 10:30 for about three nights. I almost kept her home from school (half day afternoon Kindergarten) because she was so spent. Seriously, until she was 4 1/2, she took a nap every day AND was in bed each and every night at 6:15!
DS is almost 5. He still takes a nap about 5x a week, but if he naps, he will be up until 9:30.
DD is almost 3 and takes a nap every day for about two hours. If she misses it, she definitely needs one the next day or she is completely out of control!
It probably has to do with genetics! Also, we were extremely consistent with naps and always made sleep a priority for our kids because my sister's kids NEVER slept (made her miserable) and it scared me to death! My husband works from home so there was never any problem leaving a sleeping kid home while I went out to do errands. The other thing is, my kids were never carseat sleepers. Every once in a great while they might fall asleep if we were coming home near noon, but in general they never could fall asleep in the car.