Conversation so far:
What did he do with the Rolling Stones?
Taylor's onstage debut as a Rolling Stone, at the age of 20.
What year was that?
1969.
How long did he play with the Stones?
CANNOTANSWER.
What else happened during this time?
June 1969, John Mayall recommended Taylor to Mick Jagger.

Question: What did Jagger say?
Self-contained rewrite: What did Mick Jagger say when John Mayall recommended Mick Taylor to him?