Make yourself a few years older and then book as an adult. Then send an e-mail to the ferry company (Superfast). Say that their website has a problem: you could not book the correct ticket with Interrail discount with your correct age. And then ask them to correct your age. You won't be the first to do it this way.

Really so you don’t reckon it will be a problem if I just use the wrong age? Because I have to put down my age using my passport number or some other piece of information? 

I have no personal experience here but I certainly read from a few others who did it this way. Superfast corrected their ages.

Ok I tried finding other people with similar experiences but I just couldn’t, so thank you so much 

Was it not even more simply that the discount they give to junior is the same as to any passholder=or, IOW-you gain nothing. Just check normal junior fare and adult+discount. also DO check what extra chrages may apply-thy are very keen to hide that (porttax, fuel surcharge, hi-season surcharge, onle booking charge, etc)

Yeah the junior ticket is like €68 euros whereas the adult with discount is €42 so it’s quite a lot of difference. But yeah I’ll double check everything 

They show the final price (including taxes and charges) after you've indicated your discounts, so that's clear enough.