Are there writing competitions/awards for novel length works by authors under 18?
Well yes there are awards though not as highly recognized as the major awards which are open to everyone. Also these awards are fairly localised. Normally your country has its own junior writing awards. You'll have to search on your government's site find out more about that. The relatively major awards for young authors are generally only for published books and if you win one of them you stand a chance doing having success in the normal competitions. Generally though they don't differentiate writers by age groups.