I'm sorry to hear that you're not happy with your grades, but it is still possible to do well in IB with poor IGCSE grades. Dedication and good time management is much more important than just learning the material.
I am a year younger than my peers as well and I've just finished IB1. Maturity is not determined by your age but by your attitude. How did you approach your IGCSEs? Did you really work hard and gave it your all, or were you mostly procrastinating through your course and couldn't be bothered to the work you were assigned? Your attitude is much more important than how much you actually know/knew, because if you had understood everything but just wanted to get things over with and scribbled down random stuff in your exams and coursework, you would not get a higher grade than your friend who might not be the brightest but revised constantly and did his work to the best of his ability. If you really don't want to repeat a year, then you will have to start/continue to do all you can to convince your teachers and parents that letting you start IB was not a mistake.
Of course, your grades are important to some extent, and how well you think you understood IGCSE material is often a good indicator of whether you should take it to the next level. If you desperately struggled with most of your IGCSE subjects, especially core subjects like English, maths and science (which are compulsory at IB), then perhaps you should consider repeating the year. However, the (semi-)good thing about IB is that you can choose to study the subjects you're less strong in at SL, which covers less material. Hopefully, you will be able to study in depth the subjects that you like and get through the ones you're less interested in, and that this will give you the motivation to continuously study hard.
Without having done IB, it is impossible to understand the difficulty of the course and the expectations IB and your school have of you, so if you think you can cope based on how
you found IGCSE and
your perceptions of your own attitude, I would say give it a go while retaking IGCSE in November. The first few months of IB will be relatively less stressful and should give you enough time to review IGCSE material. If, come October, you really are struggling and don't want to continue with IB, then perhaps you could ask your teacher to allow you return and repeat Year 11. Ultimately, you and you alone can make this decision, as we don't know your abilities and strengths. Whatever your decision is, good luck!
And sorry for the very long essay!