"Good morning, Auntie."
Actually, it is... If your grandparents have a child after your parents have you... you would be older than your aunt or uncle...
Let's say your grandparents have your mother when they are 16 (not impossible). If your parents have you when they are 16 (also not impossible), your grandparents would be 32 or 33, depending on exact timing. It is perfectly possible for your grandparents to then have another child.
As a language instructor for Japanese we usually teach our students that the Japanese culture places great importance on respect (as with most asian countries). Hence, when talking with people who are of higher authority/older than you, we tell them that they should use おはようございます (good morning) and not just おはよう.
Regardless if your aunt is of the same age as you or even younger. That person is still your aunt and in Japan, teachers usually speak formal to their students as well (even though they are older of course).
Of course there are some who don't really speak in a formal manner with their relatives as they are already quite close. But, you also need to take note of the cultural difference between Western and Asian cultures. While it is normal for people living int the US for example to call their Aunts/Uncles, etc. by their first names, that is not the same for the Asian culture. Again, the asian culture places great importance in showing respect for their elders.
For those saying "what if the aunt is younger than you" either way I think that you should still greet them formally at first (and you can change your tone to your casual speech should the other person state first that you don't need to be formal (you never assume it's alright to be informal just because you're the same age or just because the other person is younger). Also, when people say Aunt, more often than not they are usually older than you (aunts who are at the same age as you are or younger are usually a rare case so it's just best to go with common sense here).
"Auntie" seems like an odd translation to me. The only two contexts where I come across that word are in conversations about "Auntie Em" from the Wizard of Oz or when I buy Auntie Anne's pretzels. I always assumed it was a part of a regional dialect from somewhere (Kansas? the midWest?).