Ending Game When Turtle Reaches Edge of Grid

I have to write a program that implements the use of Turtle Graphics. I have already wrote majority of the program which includes the grid and creation of multiple turtles, but I am having a hard time ending the game once a turtle reaches the edge of the grid I created. Here is what I have so far:

import turtle
import random

# Setting up Turtle Graphics Window
turtle.setup(800,600)
window = turtle.Screen()
window.title("Turtles Walking through Grid")
window.bgcolor("black")


# Making the turtle
grid = turtle.getturtle()
grid.shape("classic")
grid.color("white")
grid.speed(10)

# Creating the Grid (Relative Positioning)
grid.penup()
grid.setposition(-300,200)
grid.pendown()
grid.forward(600)
grid.right(90)
grid.forward(400)
grid.right(90)
grid.forward(600)
grid.right(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(400)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(400)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(600)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(600)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(600)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(600)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(600)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(600)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(600)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(600)
grid.right(90)
grid.forward(40)
grid.right(90)
grid.forward(600)
grid.left(90)
grid.forward(40)
grid.left(90)
grid.forward(600)

# User Input for Speed 
speed = int(input("Enter the speed of the turtles (1-10): "))

# Variable for choosing colors
all_colors =      ["red","white","blue","hotpink","purple","lightgreen","yellow"]

# Creating the turtles
def createTurtles(turtle_count):
    count = []
    for k in range(0, turtle_count):
        lil_guys = turtle.Turtle()
        lil_guys.shape("turtle")
        colors = random.choice(all_colors)
        lil_guys.color(colors)
        lil_guys.speed(speed)
        lil_guys.pendown()
        count.append(lil_guys)
    return count

# Determine where the Turtle should stop
def off_board(self):
    x = self.turtle.xcor()
    y = self.turtle.ycor()
    return x < -160 or 160 < x or y < -160 or 160 < y

# Set Turtle Amount to 5
count = createTurtles(5)
fun = True

while fun:
    for k in range(5):
        coin = random.randrange(0, 2)
        if coin == 0:
            count[k].left(90)
        else:
            count[k].right(90)

        count[k].forward(40)

# Exit on close window
turtle.exitonclick()

The program is supposed to end once on of the five turtle have reached the edge of the grid I created.