Does aeronautical experience ever expire?
As far as the FAA is concerned, they do not expire.

As far as any (reputable) flight school goes, they will probably require a full, recent minimum 40 hours of training before they sign you off for the practical exam.

The good news is that your old training, no matter how rusty, probably means you'll pick up the basics quite quickly, and probably solo (again) faster than the average, and may very well be ready for your checkride close to 40 hours.  (The national average is a checkride around 50 to 60 hours.)