Disabling scripts

Im trying to figure out how to disable one script to another, for example i have a game manager and when the timer hits zero i wish for the ai to be enabled, but also when script starts the ai is disabled. ive tried continously to implement diffrent methods but none seem to work for me :(

 using UnityEngine;
 using System.Collections;
 
 public class GAmemanager : MonoBehaviour
 {
 
     private float theTimer = 0.0f;
     private float theStartTimer = 20.0f;
     float displayMinutes;
     float displaySeconds;
      public GUISkin skin;
     public GameObject AI;
     
     
     
     void awake()
     {
         
 
         
         // failed version = AIBehaviour ai = (AIBehaviour)AI.GetComponent(typeof(AIBehaviour));
     }
     
     void Start ()
     {
         theTimer = theStartTimer;
     }
  
     void Update ()
     {
         theTimer -= Time.deltaTime;
  
         if (theTimer <= 0) {
             Debug.Log ("Over Time");
             
             theTimer = 0;
             
             
             //initiate battle phase
             
             //failed version = AIBehaviour.enabled = true;
             
             
         }
  
         if (Input.GetKeyUp (KeyCode.G)) 
         {
             Debug.Log ("Reset time");
             
             theTimer = theStartTimer;
             
       
         }
     }
  
     void OnGUI ()
     {
         GUI.color = Color.yellow;
         GUI.skin = skin;
         
         displayMinutes = Mathf.FloorToInt (theTimer / 60.0f);
         
         displaySeconds = Mathf.FloorToInt (theTimer % 60.0f);
         
         string text = string.Format ("{0:00}:{1:00}", displayMinutes.ToString ("0"), displaySeconds.ToString ("0"));
             
         GUI.Box(new Rect(10,100,100,25),"Time: " + text);
     }
 }