This is a tricky one to answer with any degree of certainty, each little Jack Russel is different. In my life so far, I've had 5 Jack Russells and noticed a minor change in temperament at 1-year-old (emphasis on minor). My current Jack Russell calmed down much more at 3. But I have had other Jack Russels who's stamina carried through to their senior years. In short, this will vary depending on individual dogs.