Age is not a problem. Mansell won his title when he was 39. I think Fangio might have been even older. And the cars they used to drive were much more challenging than now.
Vettel's problem is not his age. His performance is a problem. Binotto makes excuses for him, because someone has to justify Vettel's giant salary to Elkann. Regarding the money he had cost them, his results are very poor. His last few campaigns were pitiful. Over 12 months without a win, entire catalogue of unforced mistakes, very poor wheel to wheel racing. He's lucky Ferrari makes habit of keeping the drivers past their prime, any other team would suck him already. He had a lot of luck so far: first to be in RB in time to get that Newey's space rockett (half the grid would win the championship in that car), then transfer to Ferrari in the best moment possible (one more year with Ricciardo in the team and Ferrari woudn't even glance in his direction). Now we are being robbed of a show due to Vettel's mediocre racecraft. Is this his last year in F1? I sure hope it's his last year in Ferrari, he's a waste of a good seat. Would any other team want him?
Renault Juniors: I have to say I'm dissapointed. There would be two of them in F2 this year: Guanyu Zhou and Christian Lundgaard. Lundgaard was 6th in F3 last year, so he didn't exactly set the world alight. His teammate in ART will be Armstrong, one of the guys who beat him in F3. Zhou finished 7th in F2 last year, so not bad, but he's clearly not Russell or Leclerc. And the grid was far from impressive last year. He might get a superlicence, but does it mean he's fit for F1? Not really. And promoting average drivers just because they are from young talents programme is not a good idea. We already have one of them on the grid, his name is Giovinazzi and he is quite enough.