So everything turned out nice in the end, apart from loss of hair and figure, lack of beauty, and your own disapproval.
I guess I'm out of fashion in my attitude ... oops, my wife is twelve years younger than I am! Aargh.
Your story seems to have only a tiny connection to Aikido, it could as well have been badminton.
Well, primarily I have to ask if his attention was inappropriate ON the mat. Off the mat is no one's business but their own. If they intended to pursue a romantic relationship, I feel that speaking to the Dojo Cho would have been the respectful thing to do since he was teaching a class. It sounds like they have a steady relationship that has been ongoing for awhile and this wasn't some sort of fling designed to make him feel better about himself at the expense of the girl's self esteem. Plus, as a woman, I generally only date men 8-12 years older than me as a rule. I've found that the way my life has gone, I work better in a relationship with a man with more life experience.
On a side note, you do realize you will despise all of your daughter's boyfriends, right? If it isn't an age issue, there will be multiple other reasons for you to decide he's not good enough for your little girl. Keeping her out of Aikido for fear she will meet an older man may seem rational to you right now, but I assure you the teenage boys she goes to school with who will inevitably ask her out will be just as distasteful to you.