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Which month and year was your Merrick Bank Card opened?
What is your current AAoA on all accounts?
I vote to keep the Merrick Card open. An AF of $36 isn't all that much. Even if you decide to close it, it should continue to report for an additional 10 years. So any impact to your credit history calculations (length and AAoA) won't be noticeable for a long time, if at all.
You could also call them and request to waive the AF, since you've been a loyal card member for ~7 years.
However, even if you close it, as mentioned it won't do anything to your AAoA for a long time. As yes, once your AMEX hits it should boost your AAoA a bit since it is quite older than your other accounts.
Also am moving to Credit Cards forum.
I think that, if the $4800 CL is a big part of your UTIL strategy, then it might be worth it to keep paying the AF. However, if it wouldn't affect your UTIL too much, I'd consider closing. It's costing money and has no rewards and likely has an exorbidant APR. The account will stay on your CRs for ~10 years, so it won't immediately affect your AAoA if you close.