It depends on the individual, and at what point they feel they can live for another human being. For some men that's 20, for others it's 50, and for some it's never.
That said, on all of the statistics stating issues that the babies of older fathers have, I've never seen one without the disclaimer, "the risk for any given couple is not significantly impacted by the man's age." They believe that the statistical correlation between older men and babies with issues probably has more to do with the fact that a very large percentage of older men who are still trying to have babies were unable to earlier in life, often because of medical issues, and that many of the older men whose children had issues also had children that they fathered at a younger age who also had similar problems. There really is no real evidence that a man's age impacts the health of a child, so an older man certainly shouldn't be precluded from fathering a child in most cases as long as he has the patience and love any child needs.