This has already been linked earlier in the thread:

https://www.aa.com/i18n/travel-info/...nd-arrival.jsp

60 minutes for international is the default, but there is a long list of airports that have different deadlines. You don't say what airport you were leaving from so it's impossible to say whether the agent was correct about the 90 minute deadline.

If you were at the desk (not just at the airport but actually talking to the agent) 60 minutes before departure and your airport is not listed as one that has a longer deadline, then you have a valid complaint and I would pursue it. If not then it's on you.