Platform.cs

using UnityEngine;
using System.Collections;

public class Platform : MonoBehaviour
{
    private Vector2 pos;
    public float speed;
    private GameObject player;

    void Start()
    {
        pos = transform.position;
        player = GameObject.FindGameObjectWithTag("Player");
    }

    void Update()
    {
        MovePlatform();
        ColliderEnabler();
    }

    void MovePlatform()
    {
        pos.y -= speed * Time.deltaTime;
        transform.position = pos;
    }

    void ColliderEnabler()
    {
        if(player.transform.position.y > transform.position.y + 1f)
        {
            GetComponent<EdgeCollider2D>().enabled = true;
        }
    }
}