"On entering the tomb they saw a young man in a white robe seated on the right-hand side, and they were struck with amazement." (Mark 16:5)

Why does St Mark, alone among the evangelists, describe the angel in the tomb as a "young man in a white robe"?

The only other time he refers to a young man is during the arrest of Jesus, and again there is a reference to the clothing being worn:

"A young man who followed him had nothing on but a linen cloth. They caught hold of him, but he left the cloth in their hands and ran away naked." (14:51-52)