What major theme does the final couplet introduce into the sonnet?
love
death
life
old age

Old age is the major theme which the final couplet has introduced into the sonnet. A person should express his love in his life before the time comes to an end. The writer has strongly contemplated to spend life fully before the time checks out.

Explanation:

The sonnet addresses the youth in context of the three quatrains which shows metaphors:The AutumnThe passing of a daythe dying out of a fireThe author talks about how each passing day affects human's life and how the leaves of a tree shedsPerhaps, the sonnet is about how someone feels of getting older and how they feel about it.it is the story of the speaker who slowly comes to the subject of the real finality of his age and his impermanence in time.

I’m not sure about maybe