Thank you for your interest and your desire to serve Israel! Thanks also for your service in the US.
At this point, it is not possible as a non-Israeli to serve in the IDF after you have turned 24. Please read the following note from the main website:
http://mahal-idf-volunteers.org/about/J ... -limit.htm
As mentioned there, though, if you really want to serve in the IDF, you can make aliyah if you are eligible as per the Law of Return.