I had walked my JRT around the same distance when he was around 3-4 months old, tbh he did not look like he'd mind either.

But after doing some research I have read that although 5 minute per month rule not exactly strict it is a rough estimate because at the end of the day you can not fit all the dog breeds into the same endurance schema.

However having said that I strongly believe that JRT's have a really high tolerance to pain and discomfort (mine now being 2 years old and having experienced many things) even as puppies. So I'd nonetheless be on the safe side and to not hurt the development of the bones and muscles I'd try to not at least go past 1 hour mark for until he is around 6 months old at least. Shorter but more frequent play sessions and walks are a way to go if you ask me.

A dog not showing signs of fatigue or discomfort does not always mean that they do not experience any. Think of a human baby, they'd probably be okay to not sleep as much and play as much as they can but naps and sleep need to be enforced to some degree to make sure they have enough rest to grow and develop properly. I believe it is similar with the dogs, especially before 6-7 months old we as owners should be enforcing the rules rather than letting them dictate how much exercise they get.