My Jack Russel is 9 years old,
and I believe she may for the first time be pregnant.
She is a fit and healthy dog,
but I'm worried that she may be too old?
Can any one give me advice please?

Take her to the vet and find out. Then get her spayed!

Any older dog, whether she's ever been pregnant or not, needs vet supervision to ensure a good outcome. So get her to the vet. And start saving for a c-section should one be necessary.