I don't remember when I started "lounging" my youngster honestly, but we did a lot of groundwork and in hand work in the round pen first. I think he was 1 1/2 maybe when I started introducing him to verbal cues in the round pen, walk on, trot, stop only. And this was very short sessions and was not about exercising but listening. I think he was about 2 - 2 1/2 when I introduced canter and used the verbal cues that I had introduced and again it was not just running in circles for the sake of running around and around. It was about communicating and listening. We also worked on join up throughout this whole time as well. He had a very light synthetic saddle on his back which weighs about 5 lbs at about 2ish and introduced him to the cinch and going for short handwalks with the saddle on. I also introduced him to the bit for short periods too. We also did a lot of ground driving in the arena and on some quiet roads and trails. At about three I was leaning over and putting some weight in the saddle and getting him used to me standing in one stirrup and really moving around. I backed him a few times at three this summer, but I really felt that he needed a few more months to develop. I don't go by an AGE per say, as it is very individual for the horse, for when he is ready for saddle training, but when they are ready mentally and physically. He has been slow at maturing. Right now he is 3 1/2 and he is pretty well developed and now ready for the trainers.
I see no rush as I want to keep him sound and healthy for many years. I have a training blog if you are interested that I kept over the years I could send it to you.