More information and maybe some pictures would help.
While it is generally frowned upon, it is your horse so...shrug. Some people start them young and have no problems at all and have sound, healthy animals for a lifetime, some people start them young and have nothing but problems. There are just too many variables at work.
That said, many QH's mature fairly quickly and are perfectly ready to start earlier than others. If she is maturing nicely and your vet gives the go ahead, she may very well be able to take short riding sessions. Heck, some of those ranch bred horses are ready to get to serious work at 2.
If you are on the smaller, lighter side, that would also make it an easier question to answer.
I guess what I'm trying to say is simply, "It Depends". Ask your vet, honestly weigh the pro's and con's with yourself and go with your gut.
/Don's flame ******ant suit...