Class call a method

That is because the method is called "changeWeight" and you are using "weight."

But what you really want is probably not a method but rather a getter/setter that acts like a property -- or maybe you just want a property.

So in your class you would have this:

package {

    import flash.display.MovieClip;

    public class Person extends MovieClip {

        private var _perHeight:Number;
        private var _weight:Number;

        private var _age:Number;

        public function Person(perHeight:Number, weight:Number) {

            _perHeight=perHeight;

            _weight=weight;

        }

        public function set weight(num:Number):void{

            _weight=num;

        }

       public function get weight():Number{

          return _weight;

       }

    }

}

Then in your code (after creating a person instance):

john.weight=180;// changes john's weight to 180.

The cool thing about getters and setters is that they act just like a property when you use them, but they allow for a whole function call at the other end and can do various things based on the inputs.

Also variable that start with an underscore are usually private variables. So in this case _weight, _age, and _perHeight would be private -- you couldn't access them directly through the john instance for example. And you would have appropriate getters and setters for them. For example you might make an age setter that didn't allow the number to be set as smaller than the current value or something.

The method would be changeWeight, wouldn't it?  And you would be assigning that new weight to the _weight variable, wouldn't you?

john.changeWeight(180);

And you should define the type in the function argument

public function changeWeight(newWeight:Number):void {

...

}