I personally feel that doing that much with a baby would just blow its mind. Most babies aren't mentally ready for that much stimulation. It'd be like taking a human child and cramming almost all of the learning it will do its entire life in the first ten or so years of life. I personally would never expose a baby to that much, most of the babies I know are left out on pasture until their two with minimal handling and by the time they are ready to start their training, even with that minimal handling, they are just as far and as responsive as horses that have been handled constantly.
Baby horses are nothing like baby humans, mentally or physically.(Besides the fact that by around 10yo humans also have
learned most of what they need to know) They are a precocial species which are designed
to learn all they need to know in the first months of life. Further to that, they are far less open to strange situations if they haven't had these experiences from a young age.
I'm not talking about 'cramming' or blowing minds anyway. I believe in doing things(at whatever age) in a non-confrontational, low stress, low pressure way. I'm not saying I'd be working at getting them excellent at x, y & z by the time they're yearlings or whatever - I think working to a preconceived schedule with horses can become a problem & can lead to over doing it.
The babies I've had the pleasure to play with have been keen, confident & required very little further training by the age of 2-3, whereas I've also had quite a lot to do with horses that have been left unhandled & depending on their personalities, they can find it quite stressful, having to learn it all then.
As far as trick training and poneying on trail rides goes, how old are we talking? Personally, I would never do this with a horse under two. It is just to much to comprehend, it can overload the foal. Again, I think it is safe to say that babies, just need to be babies.
Why is trick training, for eg. any different to anything else you would teach - picking up feet, for eg? IME the best way to get that 'well rounded horse' is to give them many & varied *good* experiences from a young age.
As for ponying for eg, I'm a hoof care practitioner & concerned also with the physical side of horses being 'left to mature' in cushy paddocks without sufficient exercise. So many horses already have pathological hoof problems by 4-6 months with this treatment, which is so unhealthy for animals designed to do many miles per day from the day they were born.