It really depends on the horse. My mare is a late bloomer, and I knew she would be when I got her. We also had me get sick and an injury to her face which prevented her wearing any kind of strap goods on it for months which slowed us down in her 4 year old year.
Her breeder showed her as a yearling, so she got some miles going places at that age. I bought her at 2 1/2, and mostly just worked on getting her to lead more like a showmanship horse than a warmblood in hand horse (more keeping even with my short legs than showing off her movement) and just getting to know each other. She learned rules like she can’t bring her shoulder in at me, and it got her ready so the day I decided to start longeing she just did it. Handling hooves, tying, etc., were part of this time.
Because she was growing and super narrow, we waited until 3 1/2 to take her to my trainer’s. He had a similar mindset to me of not longeing much because you don’t want to put that lateral torque on the legs. He did some, to work on general go forward aids, etc. He also did a lot of short rein work with her, teaching her the basics of steering both off the bit and the noseband before she had to carry a rider. He starts them all using a cavesson rein - just normal noseband with rings on the sides, and a second set of reins which you just carry on the bit when starting.
She had a time that you could see her shift mentally - she was suddenly very interested in everything everyone was doing, wanted out and to get attention, focused. My trainer rode her within a week of that, because she was just ready for it and let us know. The first ride was in a round pen so she could have her head and balance herself as needed without steering issues. She went w/t/c and there were a couple little crowhops, but that was it. I was on her by ride 5, and we kept everything very simple. She learned basics of steering with a rider, moving away from the rider’s leg as she had learned to move away from pressure from a hand on the ground, etc. Just at a walk, she started learning correct responses toward lateral work almost immediately. She also started going on trail rides ride 2 or 3. We walked a lot, and some rides we only walked. She is a very sensitive horse who can get anxious - and every ride we had what we called survival mode - just hang on and stay with her, and let her do what she needs to walk out the anxiety. The worst she ever did with me was about 3 steps sideways, but you had to be ready for any reaction and to ensure you never pulled on her mouth. Over about 6 months she got more and more confident about us, and started seeing her time under saddle as her safe place, and instead of being anxious, you’d get on and feel a big sigh of relief. At 3, she basically was ridden 2-3 times a week very lightly, nothing besides walking for over 20 minutes, but walking was as long as she needed that day.
At 4, w/t/c and some light leg yielding and large figures started getting incorporated. We’d play with lateral just to remind her of the aids, but she wasn’t ready for their gymnastic effect, and I’m a believer that if you ask for much of that when a horse isn’t ready physically you torque the legs. She also had about 6 months off because of me getting sick then her head injury.
At 5, we started leg yielding closer to what we needed for 1st level, started getting more adjustability within her gaits, working on balance and baby collection. I also started showing her training level. We still did as many walking trail rides as possible.
At 6, we started showing 1st level part way through the year, and started being able to do more meaningful lateral work. She started to have more uphill tendency especially in the canter, and more adjustability. Figures started having more gymnastic effect, and when she wasn’t in growth phases, her power started showing up some.
At 7, she found herself with the ability to sit enough to really show off some half steps, canter-walks started developing more reliably, trots started growing in amplitude and ground cover, and catching up with the canter work. We started doing more half pass work as well.
She’s 8 this year. We’ve moved up to second level, and all of a sudden she is a powerhouse at times. She can’t maintain it, but I feel the FEI horse in there part of nearly every ride. Half steps are basically available whenever I want, piaffe is developing, and the trot is developing more cadence where I can feel the passage will come with time and continuing work on adjustability. Changes were just confirmed on her - now we’re working on her rider. 
This morning when I managed to sit up properly she changed, then realized maybe if I wasn’t a pretzel I didn’t mean it, changed back, realized that I was definitely asking for the lead she changed to, and on the next stride changed back. So she has the balance and strength to change cleanly a few strides in a row. Her medium trots have recently appeared, and today we had what my trainer said was the biggest he’d ever seen from her. Our working pirouettes are getting really nice, and she has moments of insane amounts of sit in response to half halts which I didn’t know any horse could really do while still having suspension. Our lateral work is generally pretty spot on if I’m not a pretzel, as she has always found bending easy, and now she has the strength to support the carriage it calls for.
At 9 I want to show third level and get her performance bronze through the ATA - that’s many more higher scores than a rider bronze medal. Beyond that, we’ll see how quickly she progresses and is ready for the levels. Because we’ve focused so much on basics and getting things right while waiting for her body to develop, lately she seems to progress faster than expected - so if that happens and we move up faster, awesome, if not, that’s ok, too. I hope to keep riding her into her 20s.
I’m in discussion about a 4 year old who has a foal and is therefore behind “their” schedule of how quickly “they” say a horse should develop, but without a baby she’s the type who probably could have done the young horse classes and moved into FEI at “their” timeline, far faster than my mare. She was just physically developed at 2, and finds everything which has been thrown at her so far pretty easy. I shouldn’t be able to consider this horse, as she’s far nicer than I need and would have been a young horse candidate plus at least national level competitor - but circumstances are right for me to maybe get her.
