I start at 2 from the ground and 3 under saddle with low frequency. I start riding at 4 to get experience and make sure being ridden is a pleasant experience and get in some physical excercise only at 5.
They also start wearing bits only at 5 once they can do walk-canter-walk transitions, flying changes and most of what they should know in a halter.
Contact I found with my horse I lunged him one day in a set of running draw reins. So from side of girth to mouth to the bottom of the girth. He has been a cinch since. He is exceptionally smart though. Other young ones we will lunge in loose side reins so they can figure it out without a rider. Also at the standstill applying light pressure to reins. The second your horse drops head even a millimeter you give them slack in the reins and reward.
Really depends on the horse. We have one stallion that is grown and filled out at 3. He is quite small but is broke. And then we have one that is quite tall around 17 hh and he has had a hard time finding his balance and knowing where his legs are. I'd say 3 is a good age to start. Then you can always turn them out for the winter after they have been introduced to the basics. Then start them back in the spring.
But again it is totally up to the individual horse and everyone has an opinion on when to start them. Some will say to start when they are 4 or 5.