How old was Jesus when He began His ministry?

He began His ministry about age 30, as that seemed to be the minimum age to begin priesthood. His ministry lasted for just over 3 years, ending with His death on the cross.

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Numbers 4:2-3 — "Take a census of the descendants of Kohath from among the sons of Levi, by their families, by their fathers' households,
from thirty years and upward, even to fifty years old, all who enter the service to do the work in the tent of meeting.

Luke 3:23 — When He began His ministry, Jesus Himself was about thirty years of age, being, as was supposed, the son of Joseph, the son of Eli,