How can one become a sannyasin?

There are various answers in Indian traditions ("sannyas" is to renounce the world).

The problem with all answers that I know of is well illustrated by the statement below, that I have just seen: "Young, unmarried men of the Hindu religion may qualify for renunciation, called sannyasa diksha, which may be conferred by any legitimate sannyasin. But the most spiritually potent initiation comes from a satguru."

On first sight, that seems clear and understandable. But it raises key questions: what are the criteria for determining a "legitimate sannyasin"? Who is qualified to set the criteria? How do we know? What if the diksha (conferment) is done by an "illegitimate sannyasin"?

Or: Why is initiation "more potent" if it comes from a satguru? What does "more potent" mean?

Or: What are the criteria for determining who is a legitimate satguru? Who is qualified to set the criteria? How do we know?

If sannyas is something "conferred" by someone, is there a right way of conferring it which would be different from wrong ways of conferring it? Presumably the "conferment" has no "potency" if "conferred" in a wrong way? Who decides? Or how could we possibly know if it is being done right or wrong?

And what is all this about "young" and "unmarried"? I thought our traditions and scriptures were quite clear that we should all become sannyasis AFTER we have been grahasthya (householders, and therefore married) and in fact reached the age of "retirement"?