Mex Black Kings generally have the appearance of a neck. That is, the head is slightly wider than the area behind the head.
Black Milks don't usually have this. They have a wide body that just tapers to their head.
It sounds like a black king because...
1. size - a five year old snake would be adult size, or approaching it. Therefore I would expect a milksnake to be over 4.5 feet by them, depending on sex.
2. color - the brown coloration you suggest is more typical of nigrita than Black Milks. Also, a 3.5 foot Black Milk would still show faint evidence of the tricolor pattern, whereas a 3.5 foot nigrita would be solid black (brown).
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Chris Harrison
...he was beginning to realize he was the creature of a god that appreciated the discomfort of his worshippers - W. Somerset Maugham