How soon hath time, the subtle thief of youth.
Stol'n on his wing my three and twentieth year!
My hasting days fly on with full career,
But my late spring no bud or blossom show'th.
Perhaps my semblance might deceive the truth,
That I to manhood am arriv'd so near.
And inward ripeness doth much less appear,
That some more timely-happy spirits indu'th.
Yet be it less or more, or soon or slow,
It shall be still in strictest measure even
To that same lot, however mean or high,
Toward which Time leads me, and the will of Heaven.
All is, if I have grace to use it so,
As ever in my great Task-master's eye.
The speaker of this poem is John Milton. The situation is he realizes his youth is slipping away.
To me, this poem is about Milton losing his youth from the inside rather than the outside. Milton's words throughout the poem hint his getting older, such as "How soon hath time... Stol'n on his wing my three and twentieth year!" (L1-2), "But my late spring no bud or blossom show'th" (L4), and "Toward which Time leads me" (L12). Another way to look at it is time is stealing his life away in a subtle form, and his "late spring" or inner spirits show no fresh life or growth.
The aging from within shows in such statements as "Perhaps my semblance might deceive the truth... And inward ripeness doth much less appear" (L5, 7). In other words, Milton is saying his resemblance is that of youth and stamina, but on the inside much less is present. On the inside Milton feels old, and robbed of his adolescence in a way.
Milton strengthens time's power in several forms. Time is personified in Line 1 when Milton addresses it as [a] subtle thief. Continuously, line 2 gives it a physical attribute when Milton's 23rd year is stolen on time's "wing". Also, time is capitalized in the middle of line 12 to emphasize its control over Milton. This helps draw in the reader to feel like Time is a smooth criminal of young age.