Can a MRI scan determine wether a snapped ACL happened five weeks before the scan or two years before?

Yes and no. The MRI can likely determine whether the tear is acute or chronic/old. However, it cannot determine the exact age of the chronic tear....2 years or 10 years might look the same.
Most Likely Yes. A very old ACL tear can certainly be differentiated from a recent tear by MRI. 5 week old tear is subacute but I believe the differential could be made.