Hello, Mr Wong,
Stefan's answer is true as far as overseas organizations are concerned, but needs to be nuanced somewhat with respect to some dojos in Japan.
In my own dojo, for example, students become members of the Aikikai right from the first kyu rank (5th kyu, as Stefan stated), and receive a diploma signed by the Doshu. We use the Aikikai grading syllabus as a rough skeleton, but we have changed it considerably (as you could imagine with instructors who have been taught by Hombu teachers as diverse as Yamaguchi, Tada, Nishio and Asai, as well as my own teacher Kitahira). Of course, the Aikikai can be said to 'regulate' the kyu grades, but only in the same sense that they 'regulate' dan ranks, which means giving permission to people like me to hold kyu and dan examinations in accordance with my own rank.
Occasionally, I am asked by overseas instructors to look after their students during the years they are in Japan (usually US marines from the base at Iwakuni). I always accept, not least because Marines are usually very good to train (with). However, if they hold kyu grades, this means that they become members of the Aikikai much earlier than is usual overseas and so return home with one or more kyu diplomas signed by the Doshu. (Perhaps these have some rarity value
.) However, I would be surprised if these students were ever asked to take the same kyu grades again. After all, the grade was awarded by the Man Himself in the Hombu.
Of course, when these visitors come here, I generally accept their existing kyu grades, not least because they have been introduced to me by their instructor.
So, if you come to my dojo before you have have achieved dan rank, you can be sure that whatever kyu grades you receive will be Hombu kyu grades.