What did Johnny mean when he told Ponyboy to "Stay Gold"??

it comes from the the Robert Frost poem. He meant that he wanted ponyboy to retain his youthful innocence for as long as possible. The poem

He was referring to the Robert Frost poem

Nature's first green is gold

her hardest hue to hold

her early leaf's a flower

but only so an hour

then leaf subsides to leaf

so Eden sunk to grief

so dawn goes down to day

Nothing gold can stay

He recalls a poem by Robert Frost:

Nature's first green is gold

her hardest hue to hold

her early leaf's a flower

but only so an hour

then leaf subsides to leaf

so Eden sunk to grief

so dawn goes down to day

Nothing gold can stay

Then he died.