Oh, Holmes forgot to mention to Watson that Moriarty is 17. (Yes, he's a professor; he's really brilliant.)

If you arrest him when he's 17, he goes to juvie for a few days and he's out. April 27 is his birthday when he can legally be charged as an adult.

However, what even Holmes had no way of knowing was that Moriarty was born at 11:30PM. He was arrested 10 minutes shy of being 18 years old, and therefore all the charges were thrown out.