Since I've been driving my classic impreza I've noticed that some door cards seem to rattle abit and a lot of air noise coming through from outside 

 

any advise on this or is it simply because the car is 21 years old now 

They do tend to suffer from wind noise due to the doors not having framework around the glass .

Has it still got the front arch liners in situ and have you checked if the door cards have all their fixing clips intact?

Some jdm models have wind deflectors but they're fairly rare as they go brittle and snap when removed