Virtual function in virtual base class

As you have declared the function to be const in base class but not in derived class, it is not overriding.

class Animal
{
public:
Animal() { age = 0 ;}
virtual int GetAge() const { return age; }

private:
int age;
};

class Eagle : virtual public Animal
{
public:
virtual int GetAge() { age = 4 ; return age; }

private:
int age ;
};

class Lion : virtual public Animal
{
public:
virtual int GetAge() { age = 10 ; return age; }

private:
int age ;
};

class Griffin : public Eagle ,public Lion
{
}

Also the virtual inheritance does not resolve the error for ambiguous functions. You need to override the function in the most derived (Griffin) class and call the appropriate base version.