What is the best age for a Jack Russell Terrier's tail to be docked?
Jack Russell tails should be docked between 3-5 days of age, and unless you are an experienced breeder it is best if performed by a licensed veterinarian. The formula for docking tails is: take one-third OFF, leave two-thirds ON. If there are any doubts about length it is better to err on the side of too long than too short. You can always re-dock if the tail is too long but you can never add back on if docked too short.
A Myth: There is no such thing as a "standard length" of tail (i.e. 4"). The length of tail when the terrier reaches maturity will vary in relation to the size of the terrier. For example, a 10" terrier will have a shorter tail than a 15" terrier and vice versa. What is important is for the tail to be in balance with the overall size of the terrier. This is also the time to have the dewclaws removed.
The tails are docked for reasons of tradition, safety and hunting.