http://www.iantd.com/iantd3.html
I don't see "technical nitrox" listed
http://www.iantd.com/standards/IANTD_Standards_2003.pdf
This document is the entire 168 page list of standards, and will tell you everything you need to know about any IANTD class.
Not to be a jerk, but this is the kind of situation where if you have to ask questions like this, you are not ready, young Jedi.
IMHO, 15 is barely old enough for open water certification (I know, they are now beginning training at 10, and I do not agree with that). There are MEDICAL reasons why this may be unwise. Technical and decompression diving should be absolutely out of the question prior to age 18-20.
One of the primary reasons has nothing to do with maturity or frame of mind... it's a heart condition called a "PFO". This is a hole in the upper heart joining the right and left chambers. This hole allows us to survive before we are born. EVERYONE is born with it, and it generally does not completely close until we are in our teens. It is still present in about 25% of the adult population.
So what harm does it do if we all lived with it for 14-18 years?
It is a shunt, allowing blood to exchange (in small quantities) between the arterial and veinous sides of the circulatory system.
Okay... that's what it is. How does it impact decompression diving specifically?
When you are decompressing, there are bubbles forming. That's fine. That's the most efficient way to offgas. These bubbles are released and are very small, and they are released into the small capillaries. A slow ascent rate ensures that they are able to move through the gradually enlarging veinous system back through the heart, to the lungs, where they are captured by the capillary beds in the lungs and held until they dissipate. Again, a slow ascent rate allows them to dissipate before they grow to a point that they will burst the capillary beds in the lungs.
Okay... how is a PFO a problem? Those bubbles pass through the heard on their way to the lungs. The PFO allows SOME blood to pass directly to the arterial side, bypassing the lungs. This blood will carry the same bubbles. You now have a growing bubble, on the arterial side, where the arteries and capillaries are gradually DECREASING in size as they approach the extremities.
Those bubbles will not make it back to the lungs... they will be trapped and block the arterial flow.
Here's where it gets scary.
If a bubble gets trapped on it's way back to the lungs, it will most likely lodge in a joint, causing joint pain. In minor cases, it will eventually disperse, the pain will go away, and you may never realize that you were "bent".
If a bubble is trapped on the arterial side, it blocks the flow of fresh blood TO the affected area. Quite often, this is the brain or spine, and results in symptoms similar to a stroke.
Recompression may prevent further damage, but there are many ex-divers stuck in wheelchairs.
Beyond the medical reasons, experience is a main ingredient as well. I would not be comfortable beginning technical training with less than 150-200 logged dives, with at least half of those being cold-water dives in the 90-120ft range in non-ideal conditions (current, surge, etc...).
Before beginning tech training,
all of the basics need to be 2nd nature. If your mask gets kicked off, you need to be able to recover and replace it without dropping in the water column. You should also have 20-30 recreational-level dives in manifolded doubles, just to be comfortable in them.
