Yeah, a close friend of mine had a brainstem stroke in his late 40s. He's made a good recovery, but was a real mess for months after it happened, and still has balance and coordination issues and nystagmus (eyes spinning around like in the cartoons when someone gets hit on the head with a mallet). And he had no obvious risk factors before it happened, it was just a weak blood vessel that sprung a leak in a very inconvenient part of his brain. There are parts of the brain that can be removed without it being catastrophic, and other parts where loss of a cubic millimeter can kill you. The brainstem is the latter type.