koseki tohon late registration

good day, i am now 21 years old from the philippines, my father is a japanese and my mother is a filipina. many threads here are saying that i cant be put in my father's koseki tohon anymore since im over 20 years old but there is this one facebook page i saw that says it is still possible and i can obtain the certificate of eligibility for a child of japanese national visa instead of the japanese nationality so i can stay in japan. my question is, is there an age limit that prohibits a japanese national to put his child in his koseki tohon? because this is the only requirement i lack before i can apply for the said certificate of eligibility.

also, should they be married first or is it okay if they are not married to be able to put me in the family register? thank you, everyone and i am hoping for any information.

As far as I know, your Japanese father, if he chooses to, can report that he grecognizesh you as his child, and then you will be written into this gkoseki.h I suppose this is the one method that was mentioned somewhere.

When you were born, if your parents were married at the time of your birth, theyfd have had to report your birth to the Japanese authorities within 3 months, and would have also had to make the report to greserve the Japanese nationalityh for you, as you obtained the Filipino nationality at the time by being born in the Philippines (I assume).
If your parents were not married back then, then your father would have had to do a pre-birth recognition, for you to be able to obtain Japanese nationality.

But now that opportunity has been long gone, the way would be for your father to officially recognize you as his. Your parents donft have to be married now for this to happen, but whether your father chooses to do this (for example if he is married to someone else right now - it will appear in his gkosekih so his current family will know, and will also have inheritance implications later) is another question.
If you read it somewhere, please check with that group/page, and possibly ask for professional assistance to be sure. Best wishes.