Like kassierae said, it depends on the horse. I'd say you can start putting the saddle on now, it won't hurt the horse to have it on. Same with the bridle, just let the horse get used to it being in there. With both of them... put it on, take it off, put it on, take it off, etc etc etc etc. The more you make it part of her routine, the more she'll accept it as something totally normal.
There is no reason you can't teach her to lunge at the walk and maybe some light trotting on a nice long line.
Things to do with her while she's growing up.... get her used to anything and everything you can possibly think of. Big rubber balls, ropes, tarps, mailboxes, walks through the neighborhood (if you've got a safe neighborhood), car doors, jump standards and all of the random crap they put on the jumps (fake flowers, etc), cell phones, soda cans opening, seriously anything. Anything you can find should be rubbed all over her body until she no longer cares that they exist.
As for ground work-- teach her to yield her haunches & forehand. Teach her pressure on her face and poll, ask her to bend her neck around with pressure on the side of the halter just like she'd get with a bit. Teach her to stand quietly next to the mounting block while you climb it as if you were to mount her.
Just desensitize her to everything you want her to be cool with for the rest of her life.
As for riding her-- I'd say 2.5 - 3 years old, 3 being preferable. When she's used to wearing the bridle, start applying pressure to the bit as if you were in the saddle. Ask her to bend her head around to either side, when she gives, release for a reward. Ask her to back up, etc. Eventually, start to put weight in the stirrups from both sides with your arm, pushing down. Get her used to standing when there is pressure in the stirrup. Then, move to putting your foot in the stirrup, and hop up and down. flap the fenders around. Eventually you'll put all of your weight in the stirrup. Do these things over and over and over and over. When you finally get in the saddle, mount, dismount, mount dismount.
It's all repetition and it really isn't that hard. Good luck.
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