Numbers 8:24
This is it that belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service of the tabernacle of the congregation: This is it that belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service of the tabernacle of the congregation:
The customary age for Levites to enter upon their special service of the tabernacle of the congregation was twenty-five years old and upward. That age marked the normal beginning of their appointed waiting upon the tabernacle work. Samuel was an exception to this pattern, for he began to serve as a child rather than waiting until twenty-five. His early duties were fitted to his childish capacity, and as he grew he was entrusted with work of ever-increasing importance.