I'm not sure of your age, or your size, but the dojo that restricts joint-locking techniques below age 13 probably does so from an anatomy and physiology standpoint, not your maturity or your skill. i'm a family doc, so i have to admit, it's a good idea: younger students, no matter how tall, probably still have a significant amount of give in their joints, due to some areas of their bones still being soft. joint locks could inadvertantly seriously damage something before the student felt much in the way of pain in order to tap. also, if something does break in that area in a young person still growing, it may damage the growth plate, the area where the bone is lengthening. if this happens, when the break heals it may prematurely close that growth plate, leaving you with one arm or leg shorter that the other. not good. so if you are still growing, and i'll bet you are, be careful in those tecniques.
I totally agree, those techniques are dangerous no matter who you do them on, and I totally back his theory. I was not disagreeing with Clark-sensei, as I may have come off, I was just 'stickin to my guns' saying that age can be important (IE, you might not want to slam a nikyo or sankyo on a 10 year old or a 75 year old), but it should not be a basis in which to judge the person's technique.
Also- I fully understand there are many 75-year olds who _can_ handle training, it was simply an example...
Sorry for not being more clear,