How does he account for the fact that generations do not proceed in lock-step across families? Some parents have children earlier than others - siblings can be

How does he account for the fact that generations do not proceed in lock-step across families? Some parents have children earlier than others - siblings can be spread across ~15 year periods, etc. I'd hope he'd pretty easily be able to generate graphs like this for every society where the data is available - should be at least a dozen or so over the last 150-200 years. Though I'd imagine if his theory is correct, the time spans would vary slightly from society to society based on average age of parents at the time of childbirth.