Either

if you are young and bright
at one and twenty, go to a:

if you are old and gray
at one and sixty, see b:

a)
scorn these lines,
forget the book
put it lofty
things that matter
are now and here:
fun, fame, or glory
but never poetry

b)
take down this book
leaf through to see
pictures of permanent children—
they’re you
wisdom doesn’t come with age—
oh, how true