I too think it's a nervous system issue - the limp arm in combination with the involuntary narrowing of the pupils seem too coincidental for it to be anything else. However as to what caused this, it's anyone's guess. I'm not sure if frogs can have strokes but it might be possible - is it an old frog? The other possibility I can think of is disease - some diseases can result in nervous system problems, but the disease is usually apparent long before the frog starts to have nervous system issues.
Regarding the age, the larger the frog (within a gender), the older the frog is, usually.