I think that even if you do not consider the age of the horse, or the physical ability (which you must do, of course, but I'm pretending here that it's not an issue), it takes a lot of time for a horse to learn to do a Piaffe without physically restricting their forward movement while asking them to "up the energy" level. That is, if the goal is to teach the horse to do it themselves, you must take however much time it takes for the horse to learn to do it themselves, and LIKE doing it, purely as an expression of their pride in their strength.
If you have visited the website of my trainer friend, Paul Dufresne (http://www.pkequestrian.com
), you will see some new training videos that he took. These are not instructional videos, but videos he took while training his two boys, the Andalusian "Padrino", and the Friesian "Ljibbe".
When Paul was visiting our stable with his two boys, he demonstrated that both boys have a really good Piaffe in-hand (both are young, but strongly built), but this is while he restrains the head and forward movement, while encouraging the action and energy from behind as well as "mirroring" beside them.
In the videos, he is working the horses just with a neck rope - a stiffer version of a cordeo (I think it's an old Ttouch neck rope). In one of the videos (they can take a very long time to load, so be patient) he is working with Ljibbe, and he tries several times to get Ljibbe to Piaffe along the fence with just the neckrope. Ljibbe only gives it half an effort. It's a rather sad little piaffe. Ljibbe won't do it without being made to do it - this is very clear. When Paul lacks the tools he normally uses when riding and training on the ground, the horses do not collect or perform of thier own volition. This tells me they have not seen, or been shown, how delightful it can be to Piaffe. They have only been made to Piaffe.
It is the same as forcing collection. A horse that is forced to collect, will not collect himself when the force isn't present. I don't think that it is because the horse isn't capable, but rather, it is the method of training. Force does not show a horse the benefit of collection. No force allows the horse to explore the mechanics of the movement, and they find that happy place themselves where expression is allowed, and within that expression, the Piaffe can occur - from within the horse himself, and not just from within the power of the rider.
So...again, I (this is my personal thought and not based on anything scientific) don't think the age of the horse is an issue, as long as the movement is not forced. If it is simply taught and "allowed" to happen when the horse joyfully realizes it can, then age is not an issue...because the horse simply won't do it until he/she is physically capable of doing it. If that is at three years old, so be it.
Cisco is nine and physically mature. But he can't yet do it because he's either not phycially strong enough to do it, or he just doesn't yet understand how.
Tamarack will likely do Piaffe long before Cisco ever will, even though Tamarack is much younger. Tamarack is built differently and has a different point of view regarding energy and expression.