Section 1. The probate court may, if it appears necessary or convenient, appoint guardians of minors, mentally ill persons, mentally retarded persons, persons unable to make or communicate informed decisions due to physical incapacity or illness, and spendthrifts and conservators of the property of persons by reason of mental weakness, mental retardation or, physical incapacity unable to properly care for their property, who are inhabitants of or residents in the county or who reside out of the commonwealth and have estate within the county. The district or juvenile court may, when it appears necessary or convenient, appoint guardians of minors if the person who is the subject of the petition is a minor and there is a proceeding before such district or juvenile court.

For the purposes of this chapter, a mentally retarded person is a person who, as a result of inadequately developed or impaired intelligence, is substantially limited in his ability to learn or adapt, as determined in accordance with established standards for the evaluation of a person’s ability to function in society.