Numbers 4:3

From thirty - This age was prescribed, as the age of full strength of body, and therefore most proper for their laborious work of carrying the parts and vessels of the tabernacle, and of maturity of judgment, which is necessary for the right management of holy services. Whence even John and Christ entered not upon their ministry till that age. Indeed their first entrance upon their work was at their 25th year, when they began as learners, and acted under the inspection and direction of their brethren; but in their 30th year they were compleatly admitted to a full discharge of their whole office. But David, being a prophet, and particularly directed by God in the affairs of the temple, made a change in this matter, because the magnificence of the temple, and the great multitude of sacred utensils and sacrifices, required a greater number of attendants than formerly was necessary. Until fifty - When they were exempted from the toilsome work of carrying burdens, but not discharged from the honourable and easy work done within the tabernacle, Numbers 8:26. All that enter - That is, that do and may enter, having no defect, nor other impediment.