Complex sentence: He had the most interest among the band members in turning fully professional, and thus in April 1964, aged 17, he auditioned for The Who, who were looking for a permanent replacement for Doug Sandom.
Split into simpler sentences: He had the most interest among the band members in turning fully professional. In April 1964, at the age of seventeen, he auditioned for The Who, who were looking for a permanent replacement for Doug Sandom.