The max gate voltage is 18V, which is as "on" as it can get. You need to look at the graph for "Static Drain-Source On Resistance vs Input Voltage." This show's you that it starts conducting at ~5v, and the Rds(On) goes down as the voltage goes up.
Most MOSFETS are driven between 10-15v, 10v and 12v being most common on datasheets.