Albert Barnes on Mark 14:51

A certain young man. Who this was, we have no means of determining. That he was not one of the apostles is dear. It is probable that he was of one of the neighbouring villages or houses, and was roused from sleep by the noise made by the rabble, and came to see what was doing; nor is it known why this circumstance is recorded by Mark. It is omitted by all the other evangelists. It may have been recorded to show that the conspirators had instructions to take the apostles as well as Jesus; and supposing him to be one of them, they laid hold of him to take him before the high priest.

A linen cloth cast about his naked body. He was roused from sleep, and probably threw around him what was most convenient. They slept in linen bedclothes commonly, and he seized a part of the clothes, and hastily threw it round him.

The young men. The Roman soldiers, They were called young men because they were made up chiefly of youth originally. This was a Jewish mode of speaking. See Gen 14:24, 2Sam 2:14, Isa 13:18.

Laid hold on him. Supposing him to be one of the apostles.