Is it permissible to associate with Murdan (beardless youths) absolutely, or with controls and conditions, and does this include friendships between two adults with a large age difference if the same ruling as Murdan applies to them?

We do not know what you mean by "contradiction." Many scholars have warned against the companionship of beardless youths (amrad), sitting with them, and looking at them, due to the great temptation they pose. However, this does not mean that it is absolutely forbidden. If temptation is safe, there is no harm in accompanying them, sitting with them, and looking at them, especially if there is a need or an overriding benefit is hoped for. If there is no need or overriding benefit, it should be avoided, even if temptation is safe. And whoever fears temptation from sitting with, looking at, or being alone with someone other than a beardless youth, such as a bearded man or a mahram (definition), must also avoid them. It is forbidden by consensus to look at a beardless youth, a bearded man, or female mahrams with lust.