Conversation so far:
Where did Phil Mickelson go to college?
Phil Mickelson attended Arizona State University in Tempe on a golf scholarship.
How long did Phil Mickelson attend Arizona State University?
Phil Mickelson graduated from Arizona State University in June 1992.
Were there any other major golfers from Arizona State University besides Phil Mickelson?

Question: When did he move on to professional golf?
Self-contained rewrite: When did Phil Mickelson move on to professional golf?