I did not find the story unrealistic as other users have commented. Actually I know quite a few guys and girls -of different nationalities- that live or have lived with their parents not because they could not afford to live on their own but also because it is... convenient.
It is not difficult to understand why someone would still prefer to live with his parents under these circumstances - he lives as he would in an expensive and comfortable hotel (minus the bill!). Everything is taken care of by somebody else (cleaning up after him in the bathroom, laundry & ironing, bills, shopping, preparing dinner etc), he does not have to buy a car since he can use his parents' cars, he does not have to spend his money on everyday's life necessities and he can concentrate without a single worry in his mind on whatever pleases him (e.g. being an eternal student and having a different girl every night in his bed). In a few words : he's just spoiled. You can see when he tries to live on his own that he does not even make an effort, and immediately pre-judges that he could not possibly make it on his own. He does not want to live on his own and lose all the comforts he currently enjoys.
Plausibility of the story apart, I've seen the movie three times and each time I found it equally laugh-out-loud funny - a very successful combination of well-written dialogues and very good performances by the three lead actors (especially Sabine Azema and Andre Dussolier who are playing the two parents).
