There are numerous classic rock musicians, such as Don Henley, Bob Seger, and Tom Petty, who were born between 1945 and 1950, with an occasional odd bird like Roger Waters or Paul McCartney born slightly earlier (1943 and 1942). Why the cluster around those years?
It was about 1968-1978 when what we now call "Classic Rock" was simply known as "Rock." Musicians in their mid-20s to early 30s, which is the typical age of most musical trendsetters for any era, during 1968-78 would have been born in the years centered around 1945-50.
Over the course of their entire careers, most successful musicians tend to retain the sound that was prevalent during the years of their most successful recordings. Some of them were the ones who helped create that sound. Others may have came along for the ride by reflecting the same sound.
In any case, those born at "ground zero" in the post-war baby boom of 1945-50 and the few years beyond all came of age in the late 1960s and 1970s. The rapid rise in the birth rate following the end of World War II resulted in there being more young people to listen to music and, more importantly, purchase records during the time what we now refer to as "Classic Rock" had its heyday. There were also more young musicians during that time, thanks to the same demographic shifts that the "baby boom" created.
Now, fifty years later, the baby boomers are still listening to the music of their youth, as are their children who grew up hearing that same music as their parents listened to it when it was the current popular music of the day.