What age do you introduce puppies to leash pressure? 

I don't typically start leash pressure until I have great engagement and play with my own puppies. It's usually well past teething before I introduce it. But, with all things related to training, it's entirely dog dependent. It also depends on training goals and why you would need leash pressure for a puppy. I might introduce it earlier for a pet dog as opposed to a competition dog but I would not advise it for puppies in most cases.

Unless you are very experienced with training, introducing an aversive like leash pressure before the relationship and love of learning are well established can have a demoralizing effect on a puppy or young dog. Dogs do need to learn to cope with mild stress in order to have a well-rounded education but I’d always rather introduce LP later and more thoughtfully if I feel that the pup may not be developmentally ready.

The exception might be a very large and pushy pup with a small owner. It really does depend on the dog and the individual situation.