1. Any driver aged 33 or over, and they are eligible from the calendar year they have their 33rd birthday.
2. Any driver who is still in the sport 10 years following their debut, even if they are aged under 33 - this means the younger they started their F1 careers, the earlier they will be eligible for this (eg Jenson Button will be eligible from 2010 having made his debut in 2000, even though he was only 30). Also the 10 years don't have to be continous, they just have to still be in the sport 10 years after their debut.
3. I will include former champions in this if they are old enough or have been in the sport long enough, however they cannot win the Veterans and main WDC in the same year, so in the event someone eligible does with the real championship, their results for that season will be wiped out and their wins and points redistributed. (So Alain Prost will become eligible in 1989, though his 1989 and 1993 results will be nullified).
I will start from 1960 - two reasons being as the Junior Championship started in 1959, the older drivers will have been annoyed there wasn;t one for them so protested, and also the "10 year rule" will apply properly from there too.
Before I go back to 1960, I will start in the present and say who is eligible for the 2012 championship, in car number order:
Mark Webber
Jenson Button
Fernando Alonso
Felipe Massa
Michael Schumacher
Kimi Raikkonen
Pedro De La Rosa
Narain Karthikeyan
You can see if I hadn't allowed in former champions and done the 10 year rule for Massa, the field would have been rather silly - Webber and 2 HRTs. I'm trying to create something realistic here and not trying to make reject drivers winners.

Hope you enjoy!!!