Use of "this" keyword in C++

Yes, it is not required and is usually omitted. It might be required for accessing variables after they have been overridden in the scope though:

Person::Person() {
    int age;
    this->age = 1;
}

Also, this:

Person::Person(int _age) {
    age = _age;
}

It is pretty bad style; if you need an initializer with the same name use this notation:

Person::Person(int age) : age(age) {}

More info here: https://en.cppreference.com/w/cpp/language/initializer_list

It's programmer preference. Personally, I love using this since it explicitly marks the object members. Of course the _ does the same thing (only when you follow the convention)