You can't access an instance method from within an SQL computation. So you would need to refactor the method you have - most straightforwardly to a class method.
Property Age As %String [SqlComputeCode = {S {Age}=##CLASS(MyClass).GetAge({DateOfBirth})}, SqlComputed]
ClassMethod GetAge(DateOfBirth as %Date)
{
Q ##CLASS(MyDateRoutineClass).DifferenceInYears(##CLASS(MyDateRoutineClass).Now(),DateOfBirth)
}
Method AgeGet() As %String
{
...
Quit ..GetAge(..DateOfBirth)
}