There is no rule that says an Astrojax string has to be a certain length. For younger and/or shorter players, consider shortening the string. This will allow the player to perform movements without having to hold their arm in an uncomfortably high position. (As an alternative to shortening the string, younger and/or shorter players have been known to stand on a platform such as a chair, and hold their arm and Astrojax out to allow for easier play.)
To shorten the string, an end ball will need to be removed first. This needs to be done by removing the pin with a Pin Popper (see Accessories or Tuning Kits). A Pin Popper will also allow you to carefully reinsert the pin and string. Remember that the string can be shortened a bit at a time until a comfortable length is realized. If you shorten the string too much, replacement strings are available at very affordable pricing (see Accessories).
Note: Shortening your Astrojax string will naturally affect the basic orbit. A shorter string will cause faster, tighter orbits. Likewise, lengthening the string will cause slower, larger orbits.