bonus baby
n.— «Al, who signed with the Tigers in 1953, was what was then known in baseball as a “bonus baby.” As a result, he came straight to the big leagues out of high school when he was 18.» —“Big day for Kalines” by Jim Hawkins in Lakeland, Florida Daily Tribune (Royal Oak, Michigan) Feb. 27, 2008. (source: Double-Tongued Dictionary)