How to destroy the most ancient clone?

I'm spawning multiple clones from random prefabs. It works perfectly as intended if the 'spawntime' is bigger than 'lifetime'. But if 'lifetime' is bigger than 'spawntime' and multiple clones of the same prefab spawn, it looks like the old clones 'lifetime' reset as soon as the new clone is created.

Is there a way for it to not happen? So that each clone has his own 'lifetime' that never reset. Thanks.

 using UnityEngine;
 using System.Collections;
 
 public class Pill : MonoBehaviour
 {
     public Rigidbody2D bonus1;
     public Rigidbody2D bonus2;
 
     public float spawntime = 2.0f; //new clone every x second
     private float respawn;
     public float lifetime = 5f; //clone dies after x seconds
 
 
     void Start ()
     {
         respawn = spawntime;
     }
 
 
     void Update ()
     {
         respawn -= Time.deltaTime; //timer
         if(respawn <0)
         {
             int rand = Random.Range(1,2); //select bonus pill randomly
 
                 if(rand == 1) {
                     Instantiate(bonus1, new Vector3(Random.Range(-8.0f, 8.0f), Random.Range(-4.0f, 4.0f), 0), Quaternion.identity); //create bonus pill 1
                     Destroy(GameObject.Find("bonus1(Clone)"), lifetime); //destroy after its lifetime
                                 }
 
                 if(rand == 2) {
                     Instantiate(bonus2, new Vector3(Random.Range(-8.0f, 8.0f), Random.Range(-4.0f, 4.0f), 0), Quaternion.identity); //create bonus pill 2
                     Destroy(GameObject.Find("bonus2(Clone)"), lifetime); //destroy bonus5 after its lifetime
                                 }
 
             respawn=spawntime; //reset timer
 
         
         
         }
 
 
     }
 
 }