Complex sentence: When he was about 13 or 14 years old, Dr. John met Professor Longhair, which started a period in his life that would mark rapid growth as a musician and the beginnings of his entry into professional music.
Split into simpler sentences: When he was about 13 or 14 years old, Dr. John met Professor Longhair. This started a period in his life that would mark rapid growth as a musician and the beginnings of his entry into professional music.