Generally speaking, only on new growth. But the name of the variety and the type of plant (det. or indet.) can make some difference at times.

Dave

I have stems and leaves develop on the ends of clusters (stems). After a time, new flowers will develop along those ends. Part of that is due, I'm sure to the pruning I do on my plants. As digdirt said, generally speaking, only on new growth. I'm sure my results are an exception.

The succession of flowering/fruiting is upward on ANY given branch. That NO flowers will grow under any given truss. But if you happened to have a side branch below a given truss, that branch CAN grow flowers/fruits. But then that is a different branch with its own agenda. JMO

Thanks for the replies.

Is it the same for cucumbers??

And would I have to get brand new plants in order to get more fruit, or would it be possible to prune old stems & let new ones (suckers) grow?

Yes, cucumber is the same.
The only way to get flowers/fruits from an old stem is to get a new growth/side branch. Then it might flower and fruit.

Sey