While reading the athlete guide for IMMOO under USAT Rule Amendments, I see that bullet #3 states ‘Athletes wil race at the age they are on day of competition’. Do IM events not follow the ‘Age that you will be in a current year’ rule?
Nope. Different rule.
I wonder how that affects rankings?
If you do an IM at one age group are you measured against your USAT age group or your IM age group?
I need to know because it could mean I am ranked 595th instead of 607th 
For all WTC affiliated races, athletes will compete with their race day age
USAT will use your results for rankings based on USAT “age up rule” … meaning they will be adjusted in your USAT age group.