I've read the manuals. On the manual transmission there is a "reverse check mechanism" which prevents a direct 5th-to-reverse shift but there is nothing designed into the transmission to prevent a shift into 1st above any particular speed.
Try this. At 25 mph in 2nd gear, depress the clutch and shift to neutral and release the clutch. Then depress the clutch again and shift into 1st. If it won't go into 1st without grinding, take it to your dealer and get it fixed.
For reference, here's the section of the 2002 Impreza Owner's Manual that describes the operation of the manual transmission: