I put a saddle on my filly twice as a two year old, and a bareback pad once. She's only about 14hh and still has a lot of growing to do. That was back in May, she got put back out to pasture after that. I'll be going home for Christmas and I'll probably put a saddle on her again. I've laid across her back for a brief moment, but I've never really been on her. I only weigh about 100 lbs so I doubt I would do any damage. If you start them slow, they are usually fine. Walk/trot till they're past 3yrs, and only for short periods of time. I think of that first year as more for steering and softening to the bit and working off leg pressure. Simple stuff that doesn't take long to get through. She should be just fine, I wouldn't worry about it too much if I were you. I'm like you, I waited till my mare (dam to my 2yr old) was 4 before she was started. Partially b/c she's 1/2 arab and they tend to be slower when it comes to closing joints.