John Greene of Pinchbeck - Ages and procreation

I am confused. I imagine 13 year olds were allowed to marry in England around 1600 but I'm really astounded as to how a boy who died when he was 13 managed to have a son born 20 years after his death. It bothers me also that there is no source information as well.
Then there's the problem of his mother being in her late 60s when he was born. I think something it very wrong here.