We started at 25months old. She would have understood it sooner, but she was still teething and I didn't think it was fair to try to make her sit in bed waiting for the sun if she was having ew from pain, yk? I could have gone to sit with her and still used the sun as a cue to be able to leave her room in the am, but I didn't want to have to explain later why I could go sit with her sometimes (ie. When teething) but not other times.
Every kid is different though, DD1 was just rotten teether and I knew we would have ews no matter what when she had those teeth coming through.
We didn't reward, besides a "thank you/good job waiting for the sun", and even then we didn't thank or say good job for very long. It was mostly, "Sun's up, time to get up!" My style is more along the lines of no rewards for behaviour that I expect...I expect her to stay in bed until it's morning, so I don't reward it when she does, because staying in bed until morning is just the way it should be (however, we are rewarding going on the potty, not sure how we are going to stop that, lol!)