The meaning of "A Man Young And Old: XI. From Oedipus At Colonus" by William Butler Yeats is the speaker's lament of life. The poem is structured in four stanzas of triplets with an AAA BBB CCC etc. rhyme scheme. The first triplet stanza makes it clear that long life is not to be desired and that once a person grows old, the person's delight becomes longing for death. The second stanza plays off against the word "delight" but refers to the delights of fond memories, which, the speakers tells us, harbor the hidden realities of deaths, loss of hope (despair) and family estrangements etc. We are reminded that homeless wandering beggars and equally homeless and unloved children know the truth of what stanza two describes as being embodied in memories.
Stanza three contrasts stanzas one and two to a bride and bridegroom and their joyous, singing bridal party as the groom carries his bride through the dark lit only by torches and song, analogous of the looming, impending ideas brought forth in stanzas one and two. The speaker contrasts his personal celebration to their celebration saying that s/he celebrates the unreturned and silent kiss given to one who has died, making it clear that the same idea and feeling applies to one who dies young or in ripe old age. Yeats caps off this joyous sparkling poem (sarcasm there) by saying that ancient sages pronounce the best thing that could happen to a person is to never have been born; to never have breathed the air of life; never to have beheld the glories of day light. Yeats joins in the sentiment and says that, finding yourself alive, the next best thing is to enjoy the immediate moment and forget everyone as soon as you curtly and absolutely part from them--have no human bonds or affections: "a gay goodnight and quickly turn away."
To help explicate all this, I've included a paraphrase (it helped me sort through this, it might help you).
Endure the length of life you are allotted and ask for no more;
Furthermore stop thinking of the joys of your youth now that you are wearied and aged;
You always have the recourse of longing for death when longing for other things proves vain.
The delightful treasures of memory,
Harbor the unspoken of death, despair, division, and entanglements of woe,
As that person there, wandering beggar, and these children here, god-hated, know too well.
Contrasting to this beggar and these children is the dancing throng at a wedding,
The bridegroom carries off his bride to their nuptial chamber through the night lit only by torches and song;
But my celebration is the silent kiss given when a life ends, whether a long or short life.
Ancient philosophers say that it is best to never have lived;
To never have breathed, to never have seen the light of day;
So I say that the second best thing, if once forced to be alive, is a happy "goodnight" parting and to quickly turn away while remaining unattached in love or compassion.