In 1947 Miles Davis was playing in Charlie Parker's quintet, replacing Dizzy Gillespie, who had left in 1945 due to Parker's growing alcohol and drug problem. Davis recorded several albums with Parker at this time, including Parker's Sessions for the Savoy and Dial labels. By 1948 Davis had three years of bebop playing under his belt, but he struggled to match the speed and ranges of the likes of Gillespie and Parker, choosing instead to play in the mid-range of his instrument. In 1948 Davis, becoming increasingly concerned about growing tensions within the Parker quintet, left that group and began looking for a new band with which to work.
In the music business, once you hit 30 you are considered over the hill, and this still applies in the 21st century. As far as middle-aged artists go, very few make an impact on the current generation, with most inclined to live off their back catalogue. They tour the world, coining it in with merchandising sales, and endless re-issues, and deluxe packaging of their greatest hits.