The heading of this sonnet 'On his being arrived to the age of 23' is not found in either edition of 1645 or 1673.
     The sonnet has every appearance of having been written on Milton's birthday, 9 December.  And taking the usual interpretation of line 2, 'Stolen on his wing,' viz. that the 23d year is passed and gone, the date of composition would be 9 Dec. 1631.  The verses were sent to a friend, name unknown, with whom he had had a serious conversation the day before, on the subject of taking orders in the Church of England.  The friend had urged, as friends do, that it was time Milton was doing something better than 'study.'  Milton's reply is a noble vindication of the life of the intelligence, as opposed to that of action.  But Milton does not take his stand on this platform, but defends his delay on the utilitarian ground of a desire to make himself 'more fit' for life.  He wrote in the letter in which the sonnet was enclosed: 'Not the endless delight of speculation, but this very consideration of that great commandment, does not press forward as soon as many do, to undergo, but keeps off with a sacred reverence and religious advisement how best to undergo; not taking thought of being late, so it give advantage to be more fit; for those that were latest lost nothing, when the master of vineyard came to give each one his hire . . . .  Yet that you may see that I am something suspicious of myself, and do take notice of a certain belatedness in me, I am the bolder to send you some of my nightward thoughts somewhile since, because they come in not altogether unfitly, made up in a Petrarchian stanza, which I told you of.'  Then follows the sonnet, 'How soon hath Time,' &c.

     l. 1, subtle thief of youth.—Imitated by Pope, Sat. 6. 76—'The subtle thief of life, this paltry 'time.'

     l. 2, Stolen on his wing.—Pope, Im. of Martial—'While time with still career, Wafts on his 'gentle wing his eighteenth year.'

     l. 5, my semblance.—An allusion to his juvenile face and figure.  At Cambridge he is said to have been known as 'the lady of Christ's.'...  Milton tells us of himself, Defensio Secunda, that when he was forty he was always taken for ten years younger.

     l. 10, It shall be still in strictest measure even.—Nothing in Milton's life is more noteworthy than his deliberate intention to be a great poet, and the preparation he made with that intention from the earliest period.  Here we have a solemn record of self-dedication, without specification of the nature of the performance.  In 1638, we find, Mansus, 80, the determination formed, that his life work shall be a poem, though more than thirty years were to pass over before the execution of the work.