§1. A person is admitted to initial probation invalidly: 1/ who has not yet attained the age of majority; 2/ who is bound currently by a sacred bond in some institute of consecrated life or is incorporated in a society of apostolic life; 3/ a spouse, while the marriage continues to exist.
§2. The constitutions can establish other impediments to admission even for validity or can attach conditions.
§3. Moreover, to be received, the person must have the maturity necessary to lead rightly the proper life of the institute.
Source: Code of Canon Law (Vatican.va)