Not really sure about wheat but I start feeding my rirs when their old enough to be on the ground to scratch. So guessing 7 or 8 weeks plus here where I live.
As soon as they are old enough to swallow it. Mine eat some coarsely ground wheat and other grains right from the start, by a couple of weeks old they are able to handle it whole. You just need to be sure they are getting enough nutrition in their diet as wheat does not contain enough protein for growing chicks, and also, they will need grit when they are eating it.
