CSG is one of the biggest rip-offs in France. It stands for contribution sociale généralisée, and actually raises more money than personal income tax. British state pension recipients resident in France don't normally have to pay it, as the UK pays for their insurance, but early retirees normally do have to pay it, and it's a big percentage off their income.
It doesn't apply to former UN civil servants and to others in a similar position, whatever their age, as long as their retirement package includes comprehensive medical insurance. This ensures that they don't become a charge on the French system. It may be possible that private medical insurance would avoid the need to pay this charge, but you would need to check.
CSG seems, however, to be charged to all French tax residents, whatever their insurance status, on taxable capital gains and unearned income like bank interest.