You need to read the whole thing in context:
An aged man is but a paltry thing,
A tattered coat upon a stick, unless
Soul clap its hands and sing, and louder sing
For every tatter in its mortal dress,
Nor is there singing school but studying
Monuments of its own magnificence;
And therefore I have sailed the seas and come
To the holy city of Byzantium.
The whole poem sets up the contrast between the enthusiasm and artlessness of youth, and the mastery of artifice that comes with age. Yeats is basically saying that an old man is basically worthless unless he creates art out of his experiences.
@gailcalled: I read it exactly the opposite: Yeats admires the young, but his real admiration is for the monuments of unaging intellect and the sages standing in God’s holy fire, not for the young in one anothers’ arms. He doesn’t want to be young again (at least not in this poem), but he wants his body burned away so he can join the sages.