What is rhyme scheme of the following from "Ode on a Grecian Urn"? When old age shall this generation waste.

Do you mean the meter of this verse instead? You need more
lines of the poem before you can determine the rhyme scheme, which involves listening to
the sound of the last syllable in each line. For example, look at the
following:

Slowly, silently, now the
moon

Walks the night in her silver shoon,*           
*shoes

This way and that, she peers and
sees

Silver fruit upon silver
trees.

("Silver" by Walter de la
Mare)

The sounds of the last syllable in each line
(moon, shoon, sees,
trees) indicate a rhyme scheme of aabb, also
known as rhyming couplets. You simply assign the letter a to the
first sound, then b to the next sound, and so on to determine the
rhyme scheme or pattern of rhyme.

Meter, on the other hand,
is determined by listening to the stressed and unstressed syllables in a line and seeing
if there is a pattern. Your line could be read as follows with the capitalized syllables
being the stressed or accented ones.

when
OLD age SHALL this
GENerAtion
WASTE

A pattern emerges of an
unstressed syllable followed by a stressed syllable; in fact, there are five units, or
feet, of this pattern, which we call iambic. We can identify the meter of this verse
then as iambic pentameter, meaning five units of a stressed-unstressed
pattern.