// store the `this` scope in
// a variable to reuse
function Person () {
    var that = this;
    that.age = 0;
    setInterval(function growUp () {
        that.age += 1;
    }, 1000)
}

// bind the function's `this`
function Person () {
    this.age = 0;
    setInterval(function growUp () {
        this.age += 1;
    }.bind(this), 1000)
}