Ask 10 golfers how high they tee up the ball with their irons, particularly on par 3s and other shorter holes, and you'll probably get several different answers. Golfers don't all tee up the ball at the same height -- high or low.
When hitting irons off the tee, most golfers would obviously have a lower tee height than a driver. However, the question isn't tee height on its own; rather, the question is how high to tee the ball relative to the face height of the golf club being used.
Generally speaking, the best tee height with irons is to tee the ball so that there is a little room for the final one or two grooves of an iron to hypothetically slip underneath the teed up ball at impact. With irons, players are to be hitting down on the ball with a negative angle of attack, so the idea is to use your imagination here. Teeing the ball up, even just a little, is protection against impact that isn't quite flush. That could be the difference between finding the green or the fringe, or the fringe and the bunker. It also helps players that tend to sweep through impact hit the clubface in an ideal position.
Of course, some golfers feel more comfortable teeing up an iron -- any iron, really -- a little higher to try to get the ball airborne more easily. That kind of logic tends to come from how they tee the ball up with their driver. However, with irons, teeing the ball up too high could lead to decreased distance because of hitting the ball off-center.
Experiment with that placement on the range and on the course, and see how it works for your game.