You can pretty much start training from the day they are born. Get them used to ropes, pressure, and even noises behind them. Lessons in their early years should be untaxing and not asking for much forward movement for long periods. Ground driving starting at age two, then you have a year to perfect it before lightly starting them at three. If you get them in cart at three I would say only pull light vehicles or small branches/tires. Most horses should be plenty mature by age four to start being worked more for longer sessions. It is important to build your filly's trust in you and confidence before expecting her to walk out in front of you. Some horses take a while to understand the concept of walking in front of a person, or just lack the confidence to lead. As long as you do not ask too much too soon, you should be fine. If your filly is unsure or isn't confident then there is no reason you cannot continue practicing until she gets better before moving on. If she seems bored, just go over it briefly to make sure she understands then move on.
I very much agree with "foundation before specialization". Perfecting her manners with ground work is very important, and if you leave gaps in that initial training, they will come back to haunt you. Also, age doesn't matter as much as what they are mentally able to do. Some horses, even if worked from early on, may not be ready for the vehicle until years down the road. [No pun intended. ;)]
Good luck, training a horse is always a learning experience, and very worthwhile. c:
Last edited by ButtInTheDirt; 12-21-2012 at 10:34 AM.