My colt was a yearling when I got him so we did mostly groundwork the first year plus I led him in hand whenever the family went on a trail ride or ponied him off another horse so he got exposed to a lot, then when he turned 2 we introduced the saddle. We didn't get on him until he was 2 1/2 and even then, it was just my girls who weighed around 70 lbs. I didn't get on until he was 3 and then we just went on short trail rides, walking, at 3 1/2 we started doing longer trail rides and adding some trotting. Now that he's 4, we're going on regular trail rides, trotting, going up and down hills and I've taken him to one clinic. I haven't done any arena work yet but have taught him voice commands for gaits when lunging him so now that I'm ready to canter him, he knows what to do. He's still growing so I am taking it very slow and I'm sure your girl will too. I've been told they grow until 6 or 7 and the growth plates in their spine are the last to mature so be careful about getting on her back. Have you measured her to calculate how tall she should end up? Measure from the center of her knee down to the coronet band. That in inches is supposed to correspond with how many hands she'll be (example 16 in. = 16 hds). It has worked with my guy, well, it said he'd be 18 hds. And he's 17.3 at 4 so he may still grow!