What is the main theme reflected in this excerpt from “ulysses” by alfred, lord tennyson?

there gloom the dark broad seas. my mariners,
souls that have toil'd, and wrought, and thought with me--
that ever with a frolic welcome took
the thunder and the sunshine, and opposed
free hearts, free foreheads--you and i are old;
old age hath yet his honour and his toil.
death closes all; but something ere the end,
some work of noble note, may yet be done,
not unbecoming men that strove with gods