Is brain stem glioma rare? what are the likelihood of developing the disease at 17 years old?

Yes / Very unlikely: Pilocytic glioma, astrocytomas, ganglioglioma or diffuse intrinsic pontine gliomas? The majority of brainstem gliomas in the US in kids occur between the ages of 5 and 9 and total 200/year. That's about a 1 in 375,000 chance for kids in general and 1 in a billion for 17 year-olds. For every rare disease you can think to fret about I can think of 10 more. Unless you already have it, you wont get it

VERY MUCH SO: VERY RARE OCCURRENCE, AND USUALLY IN YOUNGER CHILDREN. WOULD BE EXTREMELY UNUSUAL TO PRESENT IN LATE ADOLESCENCE.