Middle aged at 28 - H

I have these Korean students who are thrilled by the fact that I am Korean and can speak a little bit. Some of them hardly speak Korean themselves, but I guess they feel some kind of connection with me. So, as they were leaving class today, they call out from the door in Korean, "Bye, Ah-jum-ma!" and of course I corrected them. "Sun-saeng-nim" (teacher) is the proper way to refer to me b/c that's what I am. I'm not really that concerned with it, except that ah-jum-ma is used to refer to older women, who in my mind are past their prime and kind of frumpy. All of the ah-jum-mas I knew growing up were older and a bit on the chubby side too. That's not me!