Silicon is immutable by default. It even has immutable data-structures, tail call optimization etc.
I have yet to show how to make anything mutable in Silicon. There is a built in Annotation for mutability.
@name age = 32;
age += 1; //error 'age' is immutable
If we really need to mutate then
@name age:mut = 32;
Technically the full type is :mut:int for mutable integer
@name age:mut:int = 32;
Syntax for mutability will likely change. age:mut[int] could be one way with type parameters (generics).