Hi Torny
The advice you have been given so far is fairly sound. I personally feel that later is better ... after all hopefully your horse will still have an active ridden life in his twenties plus!
It also depends on the type of horse that's being started. Many breeds mature late, eg some Arabs, Cleveland Bays and CB crosses etc, so would benefit from being left longer. Some people do make the mistake of thinking that because their horse is large that it is better able to cope with earlier ridden work - when the opposite is usually true as the bigger horse's ligaments, muscles and joints are often less stable than more compact types when young.
My five year old loan Arab is a good example of this as he was started and ridden away as a three year old as he was so big and strong that the then owner felt she would have problems leaving him later. My vet and physio feel that he will not completely physically mature until he is around eight or nine years old, so no (intentional) airs above the ground til then!!

We do however have nice long gentle hacks, he has started lateral work in walk and all his training will be progressive.
Hope this helps!