From USASF/IASF All-Star Cheer Divisions for 2011-2012:
The age of the competitor as of August 31, 2011 will be the age used for competition purposes throughout the 2011-2012 season for all divisions.
So it would be illegal to have a 13 yr old on the mat for a US bid competition in IO5 or IOC5.
From page 5 of
http://usasf.net.ismmedia.com/ISM2//Worlds/2012_Cheer_Worlds_Registration.pdf
All substitutes must meet the same eligibility requirements as the original team that was awarded a bid (i.e. age, etc.).
So based on that, the 13 yr old should not be eligible as a sub on the US team.
RulesGuy ?