Yeah, but if you're not living with that person......? This has been a subject that's been discussed many times. Long ago I think I said, what benefit is gained by telling someone. If there is a benefit like being able to wear at home around others, I can certainly understand that. If it's trying to gain some acceptance, I understand that need but it is pushing something private onto someone else. They probably aren't going to be ready to understand why most of us enjoy wearing and using diapers.
Like I said in another similar thread, I've thought about telling my therapist but I haven't yet. Yes, it would help him to understand another part of me and might be, but my therapy is primarily dealing with the death of my wife. I have revealed to him my having a boyfriend all throughout college and my attraction to males, at least at that time in my life. Enjoying diapers is another matter and I'm not convinced it's necessary information.
Sharing a part of us to others can be therapeutic and even liberating, but that hinges on who we are sharing with. If the OP is close to his grandmother, and if she has been someone he could always share with, maybe it is appropriate. At the least, he has opened that door.