Yes you should write the age that you will be on the day of travel. It doesn't matter much since seat reservations have a fixed price.

For normal tickets, the age on the day you travel.

For pass reservations, it doesn't matter. The reservation price for Interrail will be the same. You can also book as adult for the same price.

At the same time, I wasn't sure if there would be any problems if I book as an adult. Your answer helped with that too. Thank you :)