You are a bus driver. At the first stop, the bus driver picks up 3 people and drops off none. At the second stop, the bus driver drops of 2 people and picks up 1 person. At the third stop, the bus driver picks up 5 people and drops of no people. And at the bus drivers last stop, the bus driver picks up 7 people and drops off 4 people. How old is the bus driver?