Use of "this" keyword in C++

Possible Duplicate:
Is excessive use of this in C++ a code smell
When should you use the "this" keyword in C++?
Is there any reason to use this->

In C++, is the keyword this usually omitted? For example:

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

As opposed to:

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