LogFAQs > #974392074

So long as both the grandmother and mother were younger than 9.75 when they got pregnant (which is unusual and profoundly unhealthy, but not unheard of), they'd give birth before turning 11, which means the grandmother would be 20 at the time of her first grandchild's birth.

This is, of course, an absolutely hideous hypothetical situation that should never happen, but is not actually illegal under Colorado's laws so long as the father in both cases is less than four years older than the mother.