The age problem is a common problem of local social clubs nowadays. I joined when I was 17 and was the youngest by far. It took some explaining the the older members that freemasonry is a hobby and therefore competes with all the manifold hobbies available nowadays. We changed our approach and representation and had great success.

As others have said. The higher power can be anything as long as its genuinely important to you. I've heard of people using Darwins Origin of species or their respective Philosophical Book in the initiation. This directly represents your freedom of believing in anything and still be accepted as someone who wants to do good. But it's important to know beforehand that many of the moral texts are based on parts of the old testament. As a non Christian myself I see the stories for the message that is the core and accept the scaffolding surrounding it as such.