Well you're building their product for 1.5% or potentially 3% and the product hasn't even been validated by outside investment... I think you know the answer.
You're only 22 though, so I'd make the most out of it (learning experience).
One thing to keep in mind, is if the 2 co-founders see you as just a code monkey, chances are they don't understand how fundamental you will become to the success of the product. This means if the product becomes wildly successful you'll have an enormous amount of leverage down the road to renegotiate your compensation. If that happens, try not to be a dick about it (small world), but at the same time make sure you're appropriately compensated for bringing a successful product to market.
"One thing to keep in mind, is if the 2 co-founders see you as just a code monkey, chances are they don't understand how fundamental you will become to the success of the product. This means if the product becomes wildly successful you'll have an enormous amount of leverage down the road to renegotiate your compensation."
If they view this guy as a code monkey now, when he comes in to renegotiate they'll tell him to take a flying leap on the assumption they can just hire some random off craigslist to replace him. Assuming he's a good developer this is of course not true, but they won't know it. Eventually they will realize the terrible mistake they made when the replacement screws the pooch, but that's like 6 months later after the original developer is long gone.
If they're just becoming successful they probably won't want to rock the boat too much by firing their first employee. You could be right though if they're real douches.
It hasn't yet been validated by outside investment, but it's somewhat being validated by user tester sessions -- 3 user sessions thus far which were pretty positive (albeit they're somewhat close to the co-founders so judgement/impressions might be skewed, which is what I'm afraid of..I think they feel the same way and are looking for several outsider testers)
My dad told me the same thing, to use it as a learning experience and see how things play out over the next year...if nothing else I made 65k and it could potentially be a hit of which I own a 3% stake, worst case scenario I now have 65k in my pocket that I can use to come up with and fund my own ideas (as I have several).
You're only 22 though, so I'd make the most out of it (learning experience).
One thing to keep in mind, is if the 2 co-founders see you as just a code monkey, chances are they don't understand how fundamental you will become to the success of the product. This means if the product becomes wildly successful you'll have an enormous amount of leverage down the road to renegotiate your compensation. If that happens, try not to be a dick about it (small world), but at the same time make sure you're appropriately compensated for bringing a successful product to market.