sbool defaultAgeBasedBackupRetentionStrategy_shouldKeep(double age, double lastAge) {
  if (age <= 0.5) true; // keep all backups of last 12 hours
  if (age <= 7 && age >= lastAge+1) true; // keep every daily backup from this week
  if (age >= lastAge+7) true; // after that, keep weekly backups
  false;
}