As long as they have grit, you can give them scratch at any age. Keep in mind, it's not a complete feed so should never be the majority of the diet.
A chunk of sod from the area where their run is going to be is always a good idea. In their first few weeks of life, they are still somewhat protected by the antibodies in the yolk. This is the prime time to expose them to all the microbes in the soil so that they can build up a natural immunity, in a controlled environment. And it's just lots of fun for the chicks to eat the greens and bugs and scratch around in the dirt.