USA dollar bills never become obsolete. Banks in the US accept even the more tattered, written over and defaced bills without problems. They are sent to the Treasury to be exchanged for new bills.
Banks here accept "cara chicas", but they exchange them at the official rate. A few months ago the local cuevas stopped accepting them, or decided to discount them heavily - 6% and above.
As of last Friday, the local cueva rate was $124 for "cara grande" and $116.50 for "cara chica".