Which occasion or circumstance leads the speaker to the thoughts in Sonnet VII by John Milton?

Milton is remarking on the successes and failures of his life, looking at how old he has become because he recently turned twenty three. This caused him to reflect on his life and think about his purpose and God's influence on his life path.

" How soon hath Time the suttle theef of youth,

Stoln on his wing my three and twentieth yeer!"

These opening lines show that he feels time has been 'stolen' from him on the arrival of his 23rd year.