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That is exactly how AAoA works. Add up all the time your lines have been open, then divide by the number of lines reporting. Once you start closing accounts, they will stay on the report for 10 years in most circumstances. These lines count towards your AAoA as long as they report.
I wouldn't be totally shocked if you got a lower SL from Chase than your current Discover line; you may even find yourself looking at a $500 approval. Predicting the exact line is a fools errand for thin profiles. Regardless, the Freedom line I opened just over three years ago currently has a CL more than 20x the SL. A low start doesn't mean you're stuck there forever. Chase is a great bank to get credit with, so I'd go ahead and get started in your position.