There are many that CAN be fatal at 10 - but still have longer life expectancy past 10 in others with the same affliction. Some factors in those who die might be access to proper medical care, etc. Rather than just specifically the disease/disorder itself.
With what you state it seems like you are looking for a chronic condition that is fatal at a certain age. Rather than a critical birth defect that can kill anytime but only some MAY live until 10 (as the threshold of age limit for that critical disease). But, I cannot be certain. Can you state which it is you are going for. It may help tailor responses - as others have stated.
Are you looking for something that is a death sentence to all who have it and by 10 no matter what? Because that will be difficult with medical research and technology being what it is. Ongoing - always changing, advancing. Now in YOUR world it may be lower tech and it may work with almost ANY disease. It is hard to state with what you have offered for detail.
Cyctic Fibrosis used to be an illness that children rarely lived past 11 many years ago. Now life expectency is much longer. Same with some types of aggressive MD that boys get. Hemophilia is another that life expectency was shorter and is now longer.
If you can specifically give detail - we can specifically give response.
And try to remember - that research is great and accuracy is goal when using this type of issue as plot line. BUT, that research can only give so much on google and you will find conflicting info for almost anything you click two links for. It is hard to find information that is static across the board on anything in medical information - specifically due to advances. So, be certain of the dates on the info you read - you may be reading archived information applicable from the 60s that is moot now. That would totally ruin the effect if you have the wrong stuff to drive your MS in this part.
A good thing to do, when you finally decide on a disease, is join an on line forum for people/parents/medical professionals that have experience - real life experience - with the disorder you wish to use. They will be able to give you a perspective that research on line cannot. And please, always let them know what you are doing before you join, that it is for research purposes.
Good luck and let us know more detail so we can help you further.
Christine