In the first four lines, the narrator laments the speedy passage of time. Our youth passes so quickly, and before we know it, we have become older than we feel. He thinks that his days are flying by and that he has already reached "late spring"—the end of his youthful years. In the next four lines, the narrator admits that his appearance might make people think he is younger than he actually is, because one cannot tell how mature someone is on the inside just by looking at them. People may think he still has many youthful years left. In the next six lines, the narrator sort of gives his problems up to God, the "will of Heav'n" and the "great Task-Master." He suggests that whether he has much youthfulness left or not, no matter how quickly time seems to move, everything happens according to God's will, and so he must pray for the "grace to use" his time well. His problems no longer seem like problems when he considers that everything has been measured and apportioned out to him by God.
The speaker of the poem, presumably John Milton, is lamenting the passage of time. Specifically the problem is that the speaker feels that he has accomplished very little in his 23 years of life. He says that the days have flown by but he has nothing to show for it. "But my late spring no bud or blossom shew'th." He has entered manhood with almost nothing to show for it and is unsure of what his course should be.
The speaker's solution to that problem is to leave it in God's hands.The speaker knows that he has talents and whether or not he has a lot of time or a little time left on Earth, he wants to use that time to use his talents in a way that pleases God. "Time leads me, and the will of Heav'n [God]:/All is, if I have grace to use it so/As ever in my great Task-Master's [God's] eye. The solution makes sense because the author is John Milton who frequently wrote about God and religious topics.