Retirement is a complete myth to most agents. This might be because they are too new to even begin thinking of retirement, have settled in quite nicely and have no intention to leave at all, are too insane to consider it, have died, or believe it is a lie.
Nonetheless, some agents have managed to retire and leave the PPC. Retirement is also used as a disciplinary measure sometimes, where an agent is forced to retire from the PPC as a punishment and/or because they're psychologically unable to continue carrying out their duties. Dazey Baker's case is a good example of "punitive" retirement.