Most films in the speed range of Plus-X will go a long way past their expiration date with no noticeable defects. Plus-X, in particular, ages very well, and unless it's been stored at high temps (say, above the 80 to 85 degree range), your film should work fine.
Fog from normal atmospheric radiation will accumulate over an extended period, and shows up more in the faster film emulsions. Med speed films at just five years out of date shouldn't have accumulated much, if any detectable excess base fog. Run a test roll and see what you think. Normal developing times should work well for the age of this film. I wouldn't expect it to have lost much speed. Try your test roll at +1/2 stop, and work from there.
Dean