I think the biggest theme of this excellent sonnet is the way that the world looks on achievement and expect visible signs of having achieved something or done something with our lives, whereas actually this sonnet argues that "achievement" and "growth" can result from internal and intellectual pursuits.
If we have a closer look at the wording of this sonnet, it begins with a recognition of how quickly time flies and how the speaker has already passed his twenty-third year:
How soon hath time, the subtle thief of youth,
Stolen on his wing my three and twentieth year!
However, what concerns the speaker above all is the way in which in his "late spring" there is no evidence of having achieved or done something with his life: "no bud nor blossom sheweth." Nevertheless, the speaker argues that he has achieved "inward ripeness" in spite of the lack of evidence that he can point towards to suggest that he has been engaged in meaningful pursuits. The poem ends with a statement of belief in a God who has a perfect plan for each stage of our lives and can see both the inner "ripeness" and the outer:
All is, if I have grace to use it so,
As ever in my great taskmaster's eye.
Thus this poem is really about our lives and what we do with them. Whether we have accomplished tangible exploits that others can look to or not, this poem argues that we must not neglect our own "inner" maturity and that we should have confidence in God's plan for our lives rather than our own plan or the plans of others for us.
The person who is talking in the sonnet has just had his 24th birthday and is thinking about how quickly the first "three and twentieth" years went by. The speaker feels that the period of youth in his life is now finished and his time of adulthood is at hand, even if he doesn't outwardly look like an adult.
subtle thief of youth My hasting days fly on with full careerNow, in adulthood, the speaker is acknowledging that his life as an adult has begun and that there is no way of predicting how long it may last or when it might end: be it less or more, or soon or slow The determination of when he dies is up to God (his "great Task-Master")
I to manhood am arrived so nearinward ripeness doth much less appear