Complex sentence: Scruggs was only 15 years old then, but had undoubtedly heard of Monroe.About this time, Bill Monroe was looking for a banjo player for his group, the Blue Grass Boys, because the present banjo player was quitting.
Split into simpler sentences: Scruggs was only 15 years old then, but had undoubtedly heard of Monroe. Later, Bill Monroe was looking for a banjo player for his group because the present banjo player was quitting.