There's more to it than just jumping on their back and going, lol!
I was told not to back my mare until the age of 3-3 & 1/2. It will be closer to 3 & 1/2. I plan on starting slow. Getting her used to each piece of equipment separately, then slowly adding an additional piece throughout a few weeks. I'm going to walk her everywhere in them: in her stall (not turning her loose, mind you), in the pasture, around the house, etc. Just so she gets used to the feel of the equipment on her back, and how it moves with her/against her/etc.
Then I plan on working on mounting, and dismounting. I recently saw a Julie Goodnight video on teaching a horse to stand for mounting, and I like the method. She had the owner perform each action in small steps, and do it repeatedly, for hours. If the horse shied away at some point, or moved, they performed that action again and again until its behavior became relaxed and accepting. (I'll also have someone there to hold her, so we don't get into a train wreck.)
Once we get past that stage, I'll sit on her, but we'll still be practicing mounting and dismounting. I'll just sit on her longer and longer over the course of several weeks. Why several weeks and not several days? Because I'm in no hurry.
It might be overkill, but at least she'll be comfortable with it. Once I can comfortably mount her without her squirming or getting uneasy, I'll take away the person holding her. Once she gets comfortable mounting by herself, from different areas, I'll have a person lead me around at a walk. Eventually we'll get comfortable with leg and reining cues, and I'll walk her on my own. But personally, I don't think I'll do more than walk and trot before the age of 4. Call me paranoid, but I don't want to screw up her joints.
I'd work slowly though. Have the horse knowlegable about how to react to cues before you get to the trot. Because if your horse doesn't know the cue at the walk, he won't know what it means at the trot either!
Oh and by the way, I know exactly how you feel- I'm excited about backing my mare too!!! I can't wait for the day when we can ride around town, or even in a parade if we want to! But you have to lay the foundations first. Otherwise that trip through town, or in a parade, will be riddled with nightmares!!!