Mario games are almost always great. I don't think they're childish at all. The only Mario series that I simply don't care for is Mario Party, and I loved the first couple of them, back in the day.
Back on the topic of Metroid, the first Metroid game I ever played was the original, back in the late 80's. Got it for my 8th birthday, along with Mega Man 2. It was too difficult back then, and it's even harder now!