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.
A short explanation of "On His Having Arrived at the Age of Twenty-Three" by John Milton is that the English poet is looking on his young life so far and assessing himself. He understands that time is passing quickly - that before you know it the years stack up and we must determine what our accomplishments are and whether we are using time wisely.
Milton states that he has lived, "But my late spring no bud or blossow shew'th." In other words, have I contributed to society properly? If I have, does it show? He is not sure if he is mature yet, or maturing at the right pace, although his heart is in the right place.
The poet knows that Time will march on, God will perform his will in the Universe, and that the young man of twenty-three will, either slowly or quickly, become a productive and proper individual as intended by his Creator, if he uses what God has given him in pursuing his dreams.