This has been proven as well. A clock sent around the world on a plane came back a noticeable amount of time behind when it landed because it was traveling faster. Particles that decay at a known speed while they are stationary have been shot at very high speeds across large distances, and they decay slower than in the lab. In their frame, they are still decaying at the same speed as when they are still, but to you, the observer, they seemed squished and age slowly <- seem to age slowly means to you, they appear to decay slower than before.
It's really quite interesting. I suggest finding a higher level book on it such as an intro university text or something. A lot of layman examples are unnecessarily confusing and contradictory. Special relativity has many thought experiments that offer interesting paradoxes.
General relativity is a lot more mathy than special and requires a higher understanding of gravity and physics in general