(1) A Man over 40 years old generally, or
(2) A Muslim who is a student of knowledge. Even a new Muslim can be called a Sheikh if he is diligent in seeking the knowledge of Islam based upon Quran and authentic Sunnah. He is a Sheikh to those he can teach.
(3) Usually a person is known as a Sheikh when they have completed their undergraduate university studies in Islamic studies and are trained in giving lectures.
The word Sheikh is not to be confused with an Alim (person of knowledge).