Everybody has a different time frame they think is best to start treats.
Mine stared with plain, low-fat yogurt and some hard boiled eggs around 10 days old. Then I added crickets, but only after they found a spider in their brooder and helped themselves. After that I added a new treat every coupla days, always giving them time to make sure everything agreed with their systems.
I stongly believe in giving treats in small amounts, especially when they are little ones. You want them to be sure and get the complete nutrition of their chick starter. Treats are more for entertainment value at that age. From what I've learned since having chicks, it's never too early to start the yogurt. It has important digestive benefits to it, making it more than a simple treat.