These lines introduce the poem’s theme and create a metaphor of Time as a bird flying away with (“stol’n on his wing”) Milton’s youth.
Here, the poet expresses his sense of how quickly time passes: “hasting days” and “full career.”
The poet here uses a seasonal metaphor to express that his time of life is a “late spring” but that so far, it has not shown any “bud or blossom,” in other words any promise of fruit or achievements in his life.
The poet remarks that he does not seem as old as he is (his look “deceive[s]” the truth that he is practically a man).
“Inward ripeness” continues the natural metaphor of “bud” and “blossom” in line 4; the poet has more maturity or ripeness inside than he shows outside, and more than some other young people, the “more timely-happy spirits” have. But, note the various possibilities in the word “endur’th.” The lines are grammatically inverted and could be paraphrased, “and inward ripeness, that imbues / clothes some others, appears less in me.” The phrase “timely-happy spirits” can be understood to refer to those who are more comfortable with their age or whose age reflects more happily their inner being.
“It” may refer to the appearance of inward ripeness of line 7; whether ripeness appears less or more, now or later, it shall be just right according to his destiny, the “lot . . . / Toward which Time leads” him. Where the octave found dissonance between his inner and outward states of maturity, the sestet’s answer is that time and the will of heaven will even things out according to plan. Note the multiple puns in this line: “measure” could mean a musical measure or a line of verse; “even” may be an adjective modifying “measure” or may lead the reader into the next line, “even to that same lot.” Milton often places adjectives both before and after nouns, and he likewise often lets the word at the end of a line work in two different ways in each line.
Critics have differed as to the precise interpretation of these lines, but, in general, they suggest that whatever the outcome of the speaker’s life, it will be with God’s knowledge and in accordance with His world. The “great Task-Master” is God.