When you opened your SEP, you should have filled out Form 5305-SEP or other formal document which outlines employee eligibility requirements. What does yours say?
Instructions provided on Form 5305-SEP: "An eligible employee is any employee who: (1) is at least 21 years old, and (2) has performed “service” for you in at least 3 of the immediately preceding 5 years. You can establish less restrictive eligibility requirements, but not more restrictive ones."