Setting variable values in constructor, help

Question

Hey 

I've got this Abstract class and I want some other class to inherit from it.

  abstract class Tree {

         public string name;

 private int age;

         private int length;

         public int length {

         get { return length; }

         set { length = value; }

         }

    }

    class Oak: Tree {

        public Oak(string name, int age, int lengthInput) {

        this.name = name;

this.age = age;

        length = lengthInput; 

    }

   

The problem is with setting the value of these variables in the Oak class.

this.name = name - doesn't give me errors, because I stated in the Tree class that name is public.

this.age = age - doesn't work, it says Tree.age is inaccessible due to it's protection level.

length = lengthInput - doesn't give me errors.

So If anyone can explain to me what do these things do?

I think I maybe overprotective with my variables. When should I use private keyword? At this point I have marked all private and created get/set declarations. 

Thank you