What occasion leads milton to the thoughts in the poem?

sonnet 7: how soon hath time, the subtle thief of youth related poem content details
by john milton
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 shew'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 endu'th.
yet be it less or more, or soon or slow,
it shall be still in strictest measure ev'n
to that same lot, however mean or high,
toward which time leads me, and the will of heav'n:
all is, if i have grace to use it so
as ever in my great task-master's eye.