Milton's sonnet is a reflection about himself at the age of twenty-three. In it, he assesses himself in terms of his personal maturity, suggesting that he doesn't appear to be as mature as others of his age, but that he may be more mature than he appears. Maturity here can be interpreted in terms of achievement and accomplishment, as well as in personal growth and understanding.
The sonnet ends by bringing in the idea of divine guidance, "the will of Heaven." Milton writes in conclusion:
All is, if I have grace to use it so,
As ever in my great Task-Master’s eye.
All that he is and all that he might become, he hopes, will be a fulfillment of God's will.
Milton is thought to have written the sonnet shortly after graduating from Cambridge University, and the poem is very consistent with the thoughts of a young man who has completed the early phase of his life and is about to move into a wider world.