The Eustachian Tube or "Auditory Tube"
The auditory tube is approximately 32mm long, 2mm wide and runs from the middle ear down to an opening at the rear of the nasal cavity. Normally, the auditory tube is squeezed closed by a series of a muscles controlled both by the conscious and non-conscious portions of the brain. By remaining closed the majority of the time, the auditory tube prevents infection from traveling from the throat and sinuses up into the inner ear.
The Pharyngeal Orifice
Located just a few millimeters above the hard palate at the back of the nasal cavity, the pharyngeal orifice allows fluid to drain from the middle ear (via the auditory tube) down into the throat and stomach. Also, when a decrease in atmospheric pressure causes your inner-ear air pressure to push against the ear drum (making it feel like your ears have "popped"), yawning or swallowing will open the auditory tube. This allows the pressurized inner-ear air to escape out through the pharyngeal orifice, equalizing the pressure.
Changes In Structure
At birth, jaw and facial bones are still pretty undeveloped; as humans progress through adulthood, these bones grow and the face lengthen. Accordingly, the downward slope of the auditory tube (from the middle ear to the pharyngeal orifice) starts nearly horizontal and increases to nearly 45 degrees.
Because a child's auditory slope is relatively shallow, fluid caught behind the ear drum drains out at a slower rate. This low drainage rate gives bacteria in the fluid more time to propagate, which explains with this incidence of ear infection tend to be higher in young children.