Like the beginning of the symbolic, priestly work to cleanse the people of their sins, Christ, after His baptism begins His priestly work on earth (the outer court). Like the priests who could begin their work at age 30, Christ was baptized at age 30.
And if his offering be of the flocks, namely, of the sheep,...he shall bring it a male without blemish.
Leviticus 1:10
And Jesus himself began to be about thirty years of age,..
Luke 3:23