What should I do when my father prevents me from associating with righteous youth who help me remain steadfast in my religion, fearing a change in me that he considers a result of this companionship, knowing that his dislike for these youth is not based on any experience with them, and that I do not want to disobey him or lie to him? And what is the ruling on parents saying that marriage should not be below a certain age?

Obedience to one's parent is obligatory in matters that do not harm the child and are not disobedient to Allah. If your father forbids you from the company of righteous companions for an invalid reason, and you are harmed by losing their companionship, then you are not obligated to obey him in this matter. You may conceal the matter from him as much as you can, and it is permissible for you to use dissimulation (tawriya) and allusion (ta'rid) in such matters.