Hello
Thanks for your question, which is one I get asked a lot. There are loads of very talented school-goers out there all writing novels, something which I find impressive and inspiring. It bodes well for the future of writing and publishing!
At Greenhouse, we read everything sent into us, regardless of the author’s experience or background. However, I can understand your parents’ caution. The publishing business is a tough one and it takes resilience and strength – and a lot of rejection along the way – to become a successful writer. I suspect your parents want to protect you from that as much as possible.
Personally, I haven’t ever signed an author under the age of 18. That isn’t because I think anybody under that age is too young. It’s because, strong as some of the manuscripts from younger writers are, I haven’t yet found one that is good enough to stand out in a difficult market. It is also true that most writers improve with experience. Good authors spend their lives developing their writing and honing their craft. In my experience, it’s often a writer’s second or third book that lands them an agent and even then it can be a subsequent novel that gets them a book deal.
My advice, therefore, would be that – assuming your parents are OK with it – you send your work to agents now if you feel it’s good enough to do so (but be brutally honest with yourself on that front).  Prepare yourself for knockbacks along the way (bear in mind the countless times JK Rowling had ‘Harry Potter’ turned down) and keep at it. It may not be the novel you write at 14 that gets you an agent, but you will have nothing to lose by giving it a go. And remember, no word you write will ever be wasted. You will learn something from every paragraph.
Polly