I tend to agree with you...we all know this...BUT, it is a relatively recent thing. I belong to that group of singers (we're all within about 5 years of age) who were told in our early twenties (when the age limit was 35) to wait. At that point, there were restrictions on districts, and I think a restriction on how many times you could do the competition. The conventional wisdom was that you should wait until you were beginning to get established as a performer, and were a bit older (mid to late twenties, say). Well, then the age limit changed, and at 26 or so, you were viewed as on the old side!
I think the big "takeaway" from that should be, don't listen so much to the conventional wisdom
If your teacher is telling you you're ready, go for it.