I'm not affiliated with one. I have a 1st Dan under my father and a certificate, but I don't think his school was registered with a national governing body. There's a Hapkido school 5 mins. from my house here in Korea, but I kind of lost my mojo for it. From the ages of 15-18 though, I was really into it. I could do all sorts of flying kicks and some basic standing ones that came through for me in more than a handful of street fights. I might give Hapkido another go to get some of my dexterity back, but as of now Judo is where my passion lies.
I loved those demonstration techniques. If I tried half those flying breakfalls off of flying kicks now, I'd break my neck. Every day, jumping over the jukdo a little higher and higher. Good times.
Anyway, my primary gym and teachers were in Kunsan through my summer teenage years at a KHF, which is what I think most Yongin graduates are, but I had an uncle who handled all of my promotions since I was a kid. It made sense then, but when I think about it now, I'm not sure that's the right way to handle it.
That said, I'm not a huge fan of the standing arm-lock system in Hapkido. Those are better served for people offering passive-resistance like non-compliant civilians the police have to arrest "gently". It's super hard to wrist lock or chicken wing someone who is trying to punch you in the face. For these instances, Judo is better served.
I think you pegged that age range well, because that's when your body's flexible enough to do it and your mind is also, eh, flexible enough to buy it's efficacy. I have my own opinions regarding standing HKD armlocks and they're none too flattering. If there's anything that remained with me of value, it was the various methods of kick catching.
I haven't done or even thought about hapkido in almost ten years, same as TKD. I haven't been involved in the KMA community for a long time. I hold higher rank in TKD and HKD than I do in judo, but I think of myself as judo guy.
You're a 6th degree Black Belt?
No, judo 2nd dan. I wanted to go for my 3rd, but it became a big hassle. I paid money to USJI I think twice to get my reg numbers to compete as a shodan, but then things got screwed up because the US instructor and association I wanted to promote through was USJA or USJF. I tried twice, first time in the late 90s and second time about five years ago before I gave up.
I'm not really an active judoka in any meaningful sense anymore anyway.