Hi- I'm a vet, and none of the studies I've read have supported the idea that castration (or indeed testosterone) affects bone growth. there was always a slight question mark when it came to giant breeds, but a JRT isn't really a giant breed!
Castration is a personal choice, and there are pro's and con's
Pro's-
won't chase after/ pine after bitches in heat. The biggest proportion of road traffic accidents in dogs are unneutered males, and there is a reason for that- when they are so focussed on the scent of a bitch they notice little else- not you calling them, not a pound of sausages, not an oncoming bus! I also personall feel it's a bit harsh to leave them with all those urges and instincts intact when you know they won't be allowed to act on them. I see lots of "lovesick" pooches, who go into pining mode when the bitch 4 doors up is in season, stop eating, try to escape, are generally miserable!
Won't develop testicular cancer (not uncommon in older male dogs) Will be very unlikely to develop prostate problems and prostate cancer (one of the first things we do in prostatitis/ prostate cancer is to castrate the dog, as the testosterone accelerates the condition) Also unlikely to develop anal adenomas (tumours around the anal ring- again we often perform castration at the same time as excising these masses, as they too are testosterone dependant)
reduced sexual behaviour (obviously- although my neutered-at-6mo 15 yr old dog still humps other male dogs!) and they tend to be involved in less dog-to-dog aggression, as they are perceived as less of a threat to other male dogs.
Con's-
Involves a general anaesthetic, obviously always a small, but non-negligeable, risk
More prone to weight gain (although when people try to tell me their dog is fat "because he is neutered", I point out that all the guide dogs for the blind are neutered and you will never meet a fat one! Yes, they may need a bit less food, but it's not impossible!
Whatever you decide, good luck! FWIW I rarely castrate a dog for behavioural reasons alone- I am usually more interested in the health benefits and warn people it will NOT "calm the dog down!"