The "birthday" of television could be any of its many early points, depending on how you define "television," and it's hard to wrap our brains around how old the technology really is, since most people never saw any television until after World War II. However, it was on October 2, 1925, that Scottish inventor John Logie Baird successfully transmitted the first recognizable television image in his laboratory. He used a mechanical system that utilized the technology of German engineer Paul Nipkow to scan images of a ventriloquist's dummy's face (because its features had high contrast). Afterward, Baird scanned the face of 20-year-old William Edward Taynton to see what a real face would look like on television. The first public demonstration of his system was on January 26, 1926.
But Baird's mechanical television system was soon eclipsed by an electronic system invented by Philo T. Farnsworth, barely into his 20s when he demonstrated his system in 1928.
Television was slow to take off, first because of the competing systems, and then because it required receivers, which were too expensive for a general public that didn't understand TV. It also required programming to make the investment worth the cost. The larger radio networks worked on the problem, and Nazi Germany jumped on TV for propaganda purposes, but the rollout was halted by World War II. In the postwar boom of American prosperity, a television set became a status symbol, and TV slowly spread to almost everyone.
In the 100 years since that first transmission, television changed society, and you can explore with a mega-list of links at Metafilter.