In the recent spring, arguably the top feature is the addition of instant messaging to the xbox 360. However, many of the xbox 360's younger community will sign into their account to find quite a disappointment. To use the new instant messaging feature, you must have been at least 18 years old when you first created your xbox live account. So all child accounts, will not be able to use the messenger, period. To make things worse, you may not change your email that is connected to your xbox live account.
Note: Major Nelson has confirmed their will be an update to let users change emails, and has a small FAQ you can read by clicking on the story URL.