Complex sentence: Kirkland joined the New York City Ballet in 1968 at age fifteen after being invited by George Balanchine and was promoted to Soloist in 1969 and principal in 1972.
Split into simpler sentences: Kirkland joined the New York City Ballet in 1968 at age fifteen at the invitation of George Balanchine. She was promoted to soloist in 1969 and principal in 1972.