I'm 24 years old, boyfriend is 26. We want to go from Paris to Brussels and back. Which fare do we choose? Smilys is the cheapest, but I have no idea if we're eligible...

Contact them directly and inquire.

Everyone's eligible as long as Smilys tickets are available. There are only a limited number of seats that are sold at this cheap tariff. The tariff has some limitations: it cannot be exchanged, it is not refundable and it's always a return ticket.
With the Youth tariff (probably only valid for you if your boyfriend is 26 already) you will be more flexible because it's both exchangeable and refundable.
If your date and hour is determined, I would certainly go for a Smilys ticket.

Thanks a lot (again!) Myriam, exactly what I needed to know.

And francophile03, if you're going to post reponses such as that above, you may as well not reply at all. It's perfectly obvious that I could wake up an hour early before going to work to call Belgium to 'contact them directly', but that would hardly be convenient would it? Also, it would defeat the purpose of this forum, which is to make our experiences helpful to others. If you don't have an experience to share with, or be of assistance to, someone else, then don't post.

To mp413, there should be an email address and that's what I meant.

Sorry you thought I was being sarcastic but I was not.