My pup is about 19 months old and his leash-chewing seems to be getting worse. I think it's my version of biting my nails when I'm nervous, although he seems to do it when he's very excited/stressed out/highly stimulated.
For example, in agility class today when not actually using the agility equipment he kept chewing on his leash like a maniac.
For the record I've tried bitter apple, hot sauce, and even vick's vapor rub on the leash to no avail :-(