It's not coincidence that all this talk of kids' velocity usually centers around the age of 12.
First, "12" can be a kid who just turned 12, or a kid who is one month from 13. Huge difference. Using the "5-mph-per-year" rule of thumb, a kid who just turned 12 throwing 70 mph is exceptional, 10 mph above the "rule" velocity of 60 mph. But a 12-year-old only one month from turning 13 throwing 70 mph is now only 5 mph above the "rule" velocity of 65 mph, which is good, but not out of this world.
Second, and more important, "12" is a chronological age, that is, a birthday age, not a biological-development age. At these ages (11-14) kids vary more in their biological age than in their birthday age. A 12-year-old can be 11 biologically while his friend, also 12, can be 13 biologically, hence 2 years of biological growth and development separating them. The impact of this on pitching "velocity" is huge. Later, less so, as the biologically-11-year-old catches up and even passes the biologically-13-year-old. We've all seen this. Just look at the "12-year-old" Little League World Series pitchers each summer who throw 70 mph. Most have already had their 13th birthday (after April 30 of that year), or are about to have their 13th birthday, making them actually 13-year-olds. More, virtually all are in a state of advanced biological development, with facial hair, defined musculature, etc., making them biologically 14+ years old. A biological 14-year-old throwing 70 is not as exceptional as a "12-year-old-throwing 70!" - which is how these kids are presented on TV. He would be, rather, right in line with the "5-mph-per-year" rule of thumb: 5 mph x 14 years = 70 mph. Good, but not out of this world.