The old standby for height for outlets was about 12" from the floor. (It was common to see electricians measuring this by setting their hammer head on the floor, and placing the electrical box at the top of the handle. That was usually 12-15", and a easy, handy way for an electrician to do it consistently all thru the house.)
But new guidelines for accessible housing (ANSI Standard A117.1-1986, Uniform Federal Accessibility Standards, etc.) recommend raising this to between 18-24" above the floor.
So many electricians are switching to that height now. It makes the house more useful for disabled or aged people, and thus more saleable. (And you don't have to bend over as far when installing them -- this is helpful for many overweight electricians!)