Without answering your questions directly

here's a rundown of the average age of the field over previous seasons of F1.
1959 | 31.00 |
1964 | 29.43 |
1969 | 29.91 |
1974 | 29.85 |
1979 | 30.00 |
1984 | 29.46 |
1989 | 29.67 |
1994 | 28.68 |
1999 | 28.19 |
2004 | 26.61 |
2009 | 27.15 |
[td]Season[/td][td]Average age of drivers[/td]
(Drivers' ages calculated on 1 January of each season, to the nearest whole year. Only drivers who took part in 75% of races in a season included)
When is a driver past his sell-by date?
It varies, of course. Age need not be a barrier to competition and success in motorsport, even at a high level. Motivation often wanes as drivers get older though. A couple of drivers spring to mind who went on a season too long - Damon Hill and David Coulthard. Others, though, stopped when they still had a lot to offer - Jody Scheckter certainly, and probably Michael Schumacher too. A few years ago there seemed to be a craze for young drivers, as can be seen in the table for 2004 - Button, Alonso, Raikkonen, Massa all came into the sport at a very young age. They were a particularly good crop and have by and large retained their seats, while later arrivals - Liuzzi, Speed, Monteiro, Albers - have (to some degree) failed to impress greatly and have dropped out again - this may explain why the average age has creeped up a bit in recent years. Of course there are exceptions in both camps.
Of the current grid I don't think there are many who are there for commercial reasons. Alguersuari has apparently brought some sponsorship to Toro Rosso, but his record would probably justify the seat over any of the other young contenders even if that were not the case. Nakajima certainly is driving the Williams in order to pay for the engines, and I would say that he is now the only one who you could say fits that bill (Piquet having departed).
I suspect that new teams will try, wherever possible, to take on one experienced driver (not necessarily an old one) and one young charger. This has traditionally been the way of it, and e.g. Force India are doing much the same already. So drivers like Anthony Davidson, Nick Heidfeld, Pedro de la Rosa and even Jacques Villeneuve may be in the frame for drives alongside recruits from the junior formulae.
Who should still be driving next year? I don't think, honestly, there is much call for a clear-out: firstly because none of the old guard are disgracing themselves (Fisichella, Trulli, Webber are still outpacing their younger team mates on a regular basis); and secondly because there aren't many young drivers who are really grabbing the attention at the moment. Nico Hulkenberg and Paul di Resta I would say are drivers who deserve an F1 chance; Alguersuari and Grosjean have yet to prove themselves.