I'm not saying 'yes' or 'no'; but I wonder what lends to your impression that kids quit aikido when they become teens?
Hi there, I work with teenagers in a couple of different organizations, not just with Aikido (Boy Scouts as well). This is a symptom that happens with all different types of extracurricular activities, not just Aikido. With teenage boys we call it the "Fumes" (Car Fumes and "per" fumes!). I haven't seen it much with teenage girls though. Having two teenagers myself ( I know, it doesn't seem possible), I have seen my 14 yo son starting to get burned out from being so active ( Boy Scouts and Aikido). He has been in Aikido for several years.
Luckily my son loves Aikido! When he gets overwhelmed with activities, he will take a break for about a week, then come back to it.
They need to WANT to be there, not pushed or guilt tripped to be there. Foster that kind of environment and things will be a lot easier.