They admitted that the DS has nothing to do with the GameBoy family, and that their main motive for the release of the DS was to keep Nintendo's name out there, while they finalized the next GameBoy, due to pressure from the upcoming PSP release.
That's exactly it. The DS was designed for the under 13 crowd and the GBA and PSP family's are designed more for the 15 and up crowd. I would go even further and say the PSP was designed for 18 and up. Just my opinion though.