Is it permissible to prevent a young man in his thirties from marrying, despite his financial capability, due to his immaturity, irresponsible behavior, and lack of ghirah (protective jealousy), and the consequent fear of embarrassment and harm among his siblings? And is marriage an individual obligation (fard ‘ayn) in this case?

This condition might be a form of 'ataha' (feeblemindedness), which is a deficiency in intellect, not its complete absence. It is permissible to marry off a feebleminded person, and the guardian may be obligated to marry him off if he needs to get married. His need is known by the appearance of signs, such as approaching women. If the need appears and the guardian does not marry him off despite being able to, he is sinful. If the need does not appear, there is no sin. As for the fear of harm by marrying him off, the opposite might be true, as he might fall into indecency if he does not marry while in need.