They need to be over 1m50 and 36kg (so about 10 or 11 for the average child) not to have a booster seat or other car seat & they have to go in the back whether they have a carseat/booster seat or not. A very small adult would also technically be better off with a seat, but as they are of age they aren't the responsibility of the driver in the same way as a minor ie you can't compel an adult to sit on a booster seat but if a child (deemed to need one) isn't using one and the Gendarmes are feeling particularly picky they will throw the book at you, because they can. Mind you I had, simultaneously, 4 car seats and a baby-bucket-seat in my car, the Gendarmes thought it was v funny.
http://rsa.ie/Documents/Campaigns/Child%20Car%20Seat/french_leaflet...