You mean auditioning for SDN would defeat the purpose of graduating members out of AKB? Well, yeah, I can see that. Kind of. The thing is... I'm pretty sure the auditions mentioned are mainly for AKB...which of course would make a lot of sense. Who knows, maybe they'll find two or three more mature girls on the way and put them into SDN, together with the AKB alumni. Leaving open this option would allow older girls (let's say: 22) to audition and not be shut out of the system by default 'cause they didn't get into AKB when they were 15.