How wrong datatype deteriorates performance of C# app ?

Question

Generally we use int datatype for storing age relevant fields in C# programme.

 

We all know that age should be less than 150  (if worst scenario). Then it is better to use byte datatype for storing this values.

 

If we have 1000 variables which are relevant to age and taken as INT against byte datatype, how it affects performance of the application ??

 

 

 

Layman definition would be appreciated......