Why does it hurt in the side when you walk or run fast, and why does it go away with age?

Pain in the right side (liver) is associated with the overflow of the liver with blood and its stretching. Frequent and shallow breathing is accompanied by small excursions of the diaphragm. This dramatically reduces its suction function, as a result, blood flow to the heart worsens, it stagnates in the liver, which causes painful stretching of the capsule of this organ.

To avoid pain in the right hypochondrium while running, in most cases it is enough to follow simple requirements: eat 2-3 hours before exercise, be sure to warm up, avoid overstraining and make sure that breathing is deep enough during exercise.

If pain appears while running, then you need to reduce the load (slow down or switch to walking) and take several deep breaths and exhalations. Deep breathing allows you to accelerate the outflow of blood from internal organs (including the liver and spleen) and normalize blood circulation.

As for adults, this is unlikely to be related to age. As they get older, people stop exercising or follow simple rules, thanks to which they get only benefit and pleasure from sports.

It seems to be a stab - a spasm of the spleen, throwing out additional portions of blood to supply tissues with oxygen (one of the main functions of the spleen is just a blood depot).

And why it goes away with age, I can only guess. Most likely, one of two things happens: either a person stops going to physical education lessons with age, and passes the test in running, - accordingly, the trouble becomes irrelevant. Or a person is already doing it regularly for himself, and the body gets used to it, there is no effect "the body is dull"))