We do not know for sure the exact age of Jesus when He was crucified, but He was probably 33 years old. Here is the argument.
Jesus was baptized. But the reason He was baptized was to "fulfill all righteousness," (Matt. 3:15). He had to fulfill the legal requirements for entering into the priesthood after the order of Melchizedek (Psalm 110:4; Heb. 5:8-10; 6:20). Priests offered sacrifice to God on behalf of the people. Jesus became a sacrifice for our sin (1 Pet. 2:24; 2 Cor. 5:21) in His role as priest.
To be consecrated as a priest, Jesus had to be: 1) washed with water--baptism--(Lev. 8:6; Exodus 29:4, Matt. 3:16). 2) Anointed with oil--the Holy Spirit--(Lev. 8:12; Exodus 29:7; Matt. 3:16). Additionally, He may have needed to be 30 years old, Num. 4:3, "from thirty years and upward, even to fifty years old, all who enter the service to do the work in the tent of meeting."
Therefore we can conclude that Jesus began His earthly ministry at the age of 30. Since it went on for 3 1/2 years before Jesus was crucified, it is safe to say that He was 33 at the time of His death.