Question:

Who was the younger desciple, Mark or John?

Answer:

There isn't enough information in the Bible to say. John was old enough to be working as a fisherman with his brother and father (Matthew 4:21). Some suspect, though it cannot be proven, that Mark mentions himself in his account (Mark 14:51). But the word used for young man (neaniskos) can be broadly applied to a preteen up to an unmarried adult man prior to middle age.

If I had to guess, I would guess Mark was younger. But as I said, it is purely a guess.