[usmcobra]

Any children of such persons so naturalized (meaning those that became citizens at the birth of this country) dwelling within the United States, being under the age of twenty-one years at the time of such naturalization shall be considered citizens of the United States.....

It does define that the children of the first naturalized citizens of the United States are citizens and defines children as those below the age of Majority or twenty-one years old..

It is very clear.

[Non-Sequitur]

Any children of such persons so naturalized (meaning those that became citizens at the birth of this country) dwelling within the United States, being under the age of twenty-one years at the time of such naturalization shall be considered citizens of the United States....

No, meaning the children of those adults naturalized under the process just described earlier in that paragraph. It has nothing to do with the offspring of adults who resided in the U.S. when the Constitution was adopted. Their natural born status was grandfathered in by Article II.

It is very clear.

It is, but not in the way you claim.