If the five-year-old's mental age is also five, and the 60-year-old's mental age is 60, then they both have an IQ of 100: 5/5 x 100 = 100 (average IQ - on age level).
Booth was shot dead in a firefight with Union cavalry troops in a barn in the Virginia countryside on 26th April 1865, after having been on the run for 10 days.
There were a lot of reasons why Booth killed Lincoln. One of them was simply that (at least as George Atzerodt told it) he thought a group of plotters in New York would try to do it, and he wanted to beat them to the punch. He really liked being top dog.