In "Ulysses," Ulysses shows a restlessness in the face of his advancing years, striving for one last energetic reclamation of his earlier glory. In many ways, this feels like a willful defiance of the aging process. In it, Tennyson is simultaneously looking backwards, towards the awareness of and celebration of past glories and triumphs, as well as forwards, towards the future, by way of this last journey Ulysses would embark upon, before eventually dying.

It's an interesting conundrum, because I feel like this poem celebrates a very specific kind of retirement: in it, Ulysses retires from his kingship, passes on his responsibilities to his son. It presents a very active, very energetic vision of retirement, through which Ulysses seeks to recapture something of the person he had been, back when he was younger.

Tennyson's Ulysses, who narrates the poem, does not believe in retirement at any age. His whole life has been an adventure and a struggle, and he is not willing to give up that way of life regardless of his increasing age and declining strength. Towards the end of the poem he calls to those of his followers who are still alive after all the dangers experienced in Homer's epic poem:

Come, my friends,
'T is not too late to seek a newer world.
Push off, and sitting well in order smite
The sounding furrows; for my purpose holds
To sail beyond the sunset, and the baths
Of all the western stars, until I die.

"But man is not made for defeat," he said. "A man can be destroyed but not defeated." 

What is Ulysses' opinion of retirement in "Ulysses"?

The construction of Ulysses in Tennyson's poem is one that dislikes retirement.  Essentially, Tennyson constructs a character who cannot return to the life of domesticity.  After enduring so very much in his trials to get back home, Tennyson argues that Ulysses would have a difficult time putting this aside and returning to the life of the simple husband and father in Ithaca:  "Tennyson’s Ulysses refuses to accept a gentle death... He returns home with his men but becomes bored and leaves again."  For Ulysses, retirement is something to be dreaded because it helps to deaden the sensibilities that helped to forge and create greatness.  The idea of being retired, or remaining home, is one that Tennyson's Ulysses rejects because it denies the opportunity to touch greatness, to achieve a level of arete or glory that would never be achieved with the banal domestic life featured in retirement.  In the closing lines of the poem, this is the most evident as Tennyson's protagonist goes back out on the dangerous seas "to strive, to seek, to find, and not to yield."  This ending rejects retirement seeing it as the static life, as opposed to one of dynamic vitality.