5 month old male JRT wanting to mate with my female. Questions

How long should I wait before I allow my 5 mth old male to mate? Seems too young at 5 months. Any advice,?

2 years old at the earliest. As well both dogs should be health screened by a qualified veterinarian. This should include eye exams, hip X-rays, and more. Each breed has specific things to be screened for. Is your female also the same breed? What is your reason for wanting to breed them? It seems you don’t know much about dog breeding based on your question. I’d suggest you read up on the subject. Are both your dogs really good examples of their breed? There are too many dogs that don’t have homes. Do you need to add to that?

Good breeders don't breed unless they've got a waiting list for puppies. They don't breed unless BOTH dogs have full testing done. They don't breed unless the dogs, and their lines, have a history of being prime examples of their breeds (eg, championship lines in conformation or other breed specific working competitions) and are of sound temperament for the breed. They don't breed until they have done the research and education on what is involved in breeding.

The biggest thing they don't do is allow their dogs to breed just because the dog is showing an interest in mounting.

At 5 months old, your male is WAY too young to breed. You don't mention your female's age or breed at all which shows that you are not thinking this through. The female is the dog that will be at risk in any breeding. Too young, health issues, too old, temperament issues, etc, will all have a bearing on her ability to successfully breed, let alone raise the puppies. Focusing only on when to "allow the male to mate" is bad breeding.

Please, spay and/or neuter your dogs. There are JRTs in shelters everywhere. These dogs have been bred irresponsibly. Many have poor health, poor temperaments, etc. Don't be a part of it.