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Different airplanes can fly at different altitudes.The Concorde, for example, could fly at 60,000 feet, but most commercial airlines will not go higher than 45,000.

There reaches a point in earth's atmosphere where planes can no longer produce lift, because the air is not thick enough to support the plane, or to help produce thrust. This is why rigorous testing needs to be done on planes, to see how high it is that they can each fly safely.

Test planes and fighter jets can fly much higher, because they have rocket engines along with their jet engines, which don't need air to produce thrust. The X-15 managed to fly at 354,200 feet, thanks to its rocket engine.