Hi, Yes, Carlos remembers Eleanor even though they studied together fifteen years ago. That sentence is grammatically correct and all English speakers will understand exactly what you mean. However, you initial statement: I'm studing to my english test and I'm not sure about this sentence.... could someone help me? Should be (corrections in capitals): I'm studYing FOR my english test and I'm not sure about this sentence.... could someone help me? Hope that helps.

I'm studying FOR my English test. Carlos STILL remembers Eleanor, even though IT HAS BEEN 15 years SINCE they studied together. or... Carlos STILL remembers Eleanor, even though they LAST studied together 15 years ago.

Yes, it's correct