Motocross racing is fun cheap form of racing that anybody can do. Think your to old think again. Their is clubs that specialize in the vet riders and has age classes to 70+. In my home state there is over age 50 classes in the two major clubs and over 70 in the smaller vintage club. There is also track races and Vet series if you like being independent.
So what kind of bike should race for Vet class. How about one that you feel comfortable with. Many Vets ride 250's competitively because they are lighter and better handling than the more powerful 450 4 strokes. Here is a video of Vet riding and being competitive in the over 50 class with a Honda 250 2 stroke.
Buying a bike to get started for vet class is ranges from $1500 for a used 2 stroke that is at least a 2002 to up to $ 9000 for a new KTM 450. You can also buy new non- current bikes for about $4500-7700. Can you be competitive with older bike? The short answer is yes in Motocross it 90% percent rider. Recent REM racing in CA had Billy Musgrave going 1-4 on a YZ125 against 4 stoke 450's. In fact it is rumored he swore off 4 strokes. So find a bike your comfortable with and compete!