If the girl is 4 years older than the boy. And they are getting married next month. But mentioning their age on the nikkah papers will make alot of people question them. Specially people who will be the witness. Is it important to write age on nikkah papers for both? Or if the age written is incorrect will the nikkah b valid? Ps. The parents of both the party knows the age difference.
Islamically there's not need for a certificate or a piece of paper. The conditions for Nikah in Islam are as follows:
The groom and bride or their representatives should be present in the Nikah
The groom or bride should say that I have taken you in return for this much of Mahr and the other party should say that they accept and agree.
Two male witnesses or one male and two female witnesses should be present during the khutba of Nikah.
The witnesses should hear the whole transaction from start to end.
As for mentioning the age, it's not required neither on paper nor in words. I guess mentioning the name of the would-be couples, witnesses and the date on which the marriage takes place are enough for a marriage certificate-- unless the country you are in requires one's age to be on there as well.
Allah (SWT) knows best.