“What is a Skyhopper and how long had Luke flown one?” — Outcast
Essentially, the skyhopper is a high-speed flight training vehicle that may or may not be outfitted with a weapon system. The one common on Tatooine, which Luke refers to, is the T-16 Skyhopper from Incom. It’s a two-person sport vehicle that is both fast (with speeds up to 745.6 MPH) and highly maneuverable. It has a standard DCJ-45 antigrav generator for lift and a single ion engine for forward propulsion. Its operational ceiling is higher than most other similar crafts, and therefore can reach altitudes of 170+ miles without losing cabin pressure. It’s a light, easy-to-steer craft that is controlled on a gyro system. Not only can it turn on a credit, it can climb vertically on demand. Even more advantageously for Luke, the cockpit controls are set up very similar to those in an X-wing fighter. Fuel slugs, standard, provide power for the craft, and its computer is compatible with most astro droids.
Luke, for most of his youth, constructed model ships including those of the T-16. While there is no suggestion of how long he had been flying one, one can assume it had been some four or five years, given his age and skill. Apart from the model in A New Hope, you can also see T-16s in the Return of the Jedi (special edition) celebration scene–it flies over Mos Eisely. Also, there is one in the background of the homestead scene in Attack of the Clones.
My source was Shane Johnson’s Star Wars Technical Journal. More information in the Star Wars databank.