The themes considered in Sonnet 65 by William Shakespeare are those to do with the passage of time. They include age, youth, decay, poetry and the idea of memorial. The poet seems heavily concerned with age and the inevitability of change or even death to things that cannot withstand the onslaught of decay. He talks of inanimate natural things that are not affected, and compares his writing to those, and feels that this should be a comfort to the youth coming up. Age and death were common preoccupations in Elizabethan times as the common age for death was much lower (although William Shakespeare was luckier than some in that respect) and not only that, but the idea of what happened to people after was much more terrifying than for us today. People imagined the pain and horrors of the fires of hell awaiting them if they had not lived a virtuous enough life, so we should not be surprised at the poet's obseesion with aging, decay and dying.
He even looks for an escape
"O fearful meditation!" he cries, where can the young man hide that time won't wreak on him the same "siege of batt'ring days?"
yet he knows there is none - save perhaps what a man of the church might have told him which might be to save up treasures in heaven which cannot corrode and where he and they wiould be safe for ever. In this case, Shakespeare wishes more to rely on himself and his own talents and gifts, hoping that his poetry will stand the test of time and be a fitting memorial to him
"That in black ink my love may still shine bright."
and it is true that we still benefit from it today, so he was right in that.