LEC
Opinions are like bum holes, everyone has one.
As I mentioned a key issue is people expect too much from horses who are bred for a very different job than the one the owner wants them to do. Your horse was designed to pull a plough/cart 5 days a week and take them to church on a Sunday if a pure ID so isn’t really designed to jump hence they have been refined by being added to TB over the years. ID also have huge issues with canter though this has been moderately improved over the years by lightening and refining the breed. So ultimately every time someone expects something more from the way it was bred they are putting it under pressure. Everytime you canter in the school for instance as it was designed for straight lines in walk and trot. So sure take your time as you would break your horse.Personally, I hate seeing babies jumping over jumps in the school at under 5when they are being advertised for sale. Any horse can jump and to see babies jumping in an advert puts me off. Also hate the stupid music people add, so I turn sound off as I like to hear the horse when a video is being shown not hidden sound with some ghastly music.
To answer your question, I wise person told me not do anything with her till at least 6 and not serious with my ID till she is at least 7, that is my guide line, when bones are knitted more and ligaments are stronger. MY current ID was jumping show jumps before 5 years and now she has clicking in her joints. Also girl I knew was jumping her horse at 4 by 7 he was PTS as hocks gone.
Obviously people do what they want but that is my age guide.
Do it while they young and their legs get shot.
Clicking in the joints has scientifically proven not to be an indicator of arthritis.
Anecdote doesn’t equal evidence. Your friends horse could have had crap confo, crap upbringing etc.