While your question is short, it is very broad. The best answer I could give you, is it depends.
You have not really defined the attack, nor much about the nature of your attacker (though you do ask if it would make a difference if your attacker was 18 or older).
In California, the key to self defense is whether a reasonable person would have been in reasonable fear of harm had he or she been in your shoes. You have not expressed, in your hypothetical whether you personally are in any fear of this attack or whether you think most people would be in fear. In most cases, in either a civil or criminal proceeding, a jury will be asked to evaluate whether you acted as a reasonable person would have acted when confronted with the attack.
Fortunately, most reasonable people respond to an unprovoked attack by defending themselves.
The next question that would need to be addressed is the amount of force you used. Again, in California, you are not allowed to use excesseive force, and once you have disabled your attacker any further use of force against him or her would be viewed as you committing battery upon the attacker (in essence, your roles would reverse).
In your hypothetical, you only used one technique (what a traditionalist). So now ask yourself, was this technique excessive. Looking past the criminal proceeding to the civil, how will your respond at your deposition when the attacker's attorney begins to question you about your training and your knowledge of techniques? For example, when the attorney asks, "Mr. Pantazi, based upon your 90 years of self defense training, did you have other techniques available to you that would have allowed you to defend yourself against my client without causing him this serious injury?" "Mr. Pantazi, did you choose this technique because you knew it would permanently injure my client.?" "Mr. Pantazi, are there other less lethal techniques you could have used against my client?" And on and on.
The focus of your deposition will probably be all the techniques you could have used that might have caused lesser harm. And, while you may be thinking this begs the point of whether you acted as a reasonable person would have (because it does) your attacker's counsel will be attempting to elicit responses that can be used latter to paint you as a martial artist looking for a fight. Your attorney will, of course, object to much of this questioning, but in civil litigation your attacker's counsel will generally be given broad latitude to delve into your training, experience, and motivation. When thinking civil litigation, think O.J. No guilty verdict in the criminal, but ordered to pay in the civil.
While I understand the concern of your hypothetical, you might elicit more responses if you narrow down the questions.
My own concerns, for example, focus less on confrontation and more on such issues as:
I have just shown a technique to a beginning student. He now informs me he is going to use this technique to harm is wife. He is serious. Do I have a duty to warn his wife or to take other steps to prevent this student from using this technique inappropriately?
I am a corner judge in a tournament. There is a no head contact rule. The center judge is consistently allowing head contact. A participant is injured. Do I have any liability for this injury?
I am teaching a class when I notice the arts and crafts group has left pins on the floor. Do I stop the class?
I am teaching a class when I notice the floor has some new splinters. Do I stop the class?
I am teaching a class when I notice the new eager maintenance person has put high gloss and extremely slippery wax on the floor. Do I stop the class?
I am teaching a class when I notice the ballet group has left dance resin all over the floor ... well you get the picture.
Oh, and what about issues of dojo or class accessibility to the disabled. Or concerns about blood spills and exposure to wounds. Is there anyone in the class trained in emergency first aid, and if not, should the class go on?
Well, I know I would like to hear from some other attorneys on your question. (I am not sure, but I think in Texas there is actually an attacker season, where if you have the necessary permits ... but then again I could be wrong.) Peace.