On the topic of Kirby's age, I always thought of him to be around 6-8 years old. I don't see him as super young, since he doesn't seem to act like a baby, but he's not too old since he's still pretty childish in that all he really cares about is food most of the time. If he were a baby, I don't think he'd be as heroic and willing to help others as he currently is, he has to at least be old enough to know right from wrong and have a sense of justice.
Also, I'm a bit confused why people don't think Kirby is a big franchise. Isn't it more successful than Metroid, Star Fox, F-Zero and many other Nintendo franchises? It may not be super popular (at least not where I live), but you can't deny that it's a very successful Nintendo franchise and is still alive and well. Something a series like Metroid can't say. (I like Metroid, I'm just making a comparison.)