Maybe not so dumb
It's easy to snicker at this sign. It only has three lines, and two of them are in direct contradiction to each other.
But then, if you think about it, you might realize a few things:
a. it's a restaurant, not a literary agency, so the fact that they are poor at putting together sentences doesn't hurt their branding so much, and
b. you thought about it.
By creating this error (and there's no doubt in my mind it was an accident) they called attention to the fact that seniors and 16 and 17 years olds are welcome to work there. They've dramatically increased the size of the potential worker base. Stupid sure, but not so dumb.