What's acceptable as a broke horse?

It's not actually about her age but about the riding miles she's had on her. If your the only one riding her, she could have been acting like that because she was confused what was going on. Imagine hanging out with a friend a few times a week for most of your life and then all the sudden, one day a new friend shows up to hang out with you...would be uncomfortable and you wouldn't know really what to do right? You'd probably want to leave. That is one possibility. The other is that your parents may not be that good of riders or they are good riders and she was testing them to see what she could get away with--very common for horses. She's a mess when other people ride her because every rider is different and she's used to your signals. Whoever got on her may have been in her mouth or had too strong of leg contact or not enough. Your horse in not crazy, she's not trying to lash out, she's simply confused. The only way to fix this is to HAVE other people ride her and you need to be there for it. Put a good rider on your horse, have her attached to a lead rope, and just WALK the person and horse around. Make sure you reward her for every good step she takes and talk her through this. Next, put the person and horse on a longe line. You'll still be attached to her but it will be the riders job to steer her. You should tell the person how you ride her and help them along.