A lot of us when we are young tend to sleep on our stomachs and are comfortable that way. Then when we get older, it's just not as comfortable and we start sleeping on our sides or back. The only reason I can think for this, is that when we are young, we're little - we don't weigh very much. This means that our body weight does not sink us very far into the bed, so we remain relatively flat, a position which the body has no problem with.
When we get older and heavier, however, the body sags in the middle of the bed due to the difference in pressure near the torso. In this position, the back is bent slightly backwards, and the opposite ends of the body are also pressing against the bed with uncomfortable force. This position is unsuitable for sleeping conditions, so we favor sleeping on our sides, where the body rises off of the surface of the bed more, and back bending is less of an issue.
If you have a spring mattress, little things like your heart beating can reverberate through the mattress and make its way to your ear pressed against the pillow, which can be distracting.