1. I always knew I was different, but I knew for sure what IT was at about 12 or 13.
2. I don't know if I was diagnosed or not, but I do remember talking about it quite extensively during most of my teen years with a therapist. Whether he put pen to paper and officially diagnosed it or not I don't know. At least for now, there is a seperate diagnosis for childhood GID and teenage and adult onset GID becuase most young children still outgrow their crossgender behaviors.
3. There doesn't appear to be any risk in delaying puberty. I think its a fine idea if there's any chance the child might later be interested in full HRT. As most people here have said, the onset of puberty was quite distressing, at least being able to put that on hold while the child sorts out their feelings should be no different then giving them anti-depressants, etc.
4. I think the standards are still 18. I think a case by case basis is the best approach, but becuase of legal issues, the age of majority is safest for medical practitioners. That I'm aware, when they are approve for under 18s the target age appears to be around 14, just like everything else there are medical issues involved, including growth milestones that have to be reached.
5. I would want my child supported in the best way possible. I think a lot of well meaning parents are pushing their kids with moderate cases of GID into a full blown transition. There are examples where it was either suicide or letting your third grader attend school as their target gender; but I get the sense there are alot of well meaning parents that take steps to support their child that end of giving them little to no means of going back to their birth gender should they change their minds.