Why is this simulated transistor turning on at such a low voltage?
I am not 100% sure what is going on here.  Turn-on is a gradual process, but I agree that your simulation seems to not match the datasheet.

However, you are simulating this MOSFET pretty far outside its "normal operation" window.  This is a fairly high current MOSFET (17 amp continuous), and you are using it in a circuit that only draws a maximum of about 200 uA.  For instance, the leakage current at 25 C is 25 uA.  At 150 C, the max leakage is 250 uA!  I could believe that the transistor model in multisim is not great when you are trying to operate near the leakage current level.

Try decreasing R2 or changing to a more suitably sized MOSFET and see if it works better.