“Do we follow all the other dictates of this Mishna? Are 5 year old kids learning Mikra for five years, then Mishna for five years, then start Gemara at 15? No.
The Mishna is giving us suggested guidelines that our present day Chachamim will interpret and apply as needed.”
And the Conservative Movement says that they can interpret and apply mishnayot as needed. There has to be a better answer to them than “we are right and they are wrong”.
And a good argument can be made — in fact it has been made, by Rabbi Hershel Schachter among others — that boy’s yeshivot overemphasize gemara, starting too early and ignoring the rest of our mesorah. The author of this mishnah would likely have agreed.