Check out these proposed adjustments to the Federal Register from a few years ago. Part of the adjustments, section E talks about adopting a clearer definition of "model year". This is for import eligibility, not for 25 year old cars, but it does set aside some clarity from the point of the NHTSA.
E. Adopting a Clearer Definition of the Term “Model Year” for the Purpose of Import Eligibility Decisions
When an import eligibility petition is based on the substantial similarity of the subject vehicle to a U.S.-certified counterpart, section 30141(a)(1)(A) requires the agency to make the eligibility decision on a model and model year basis. If there is no substantially similar U.S.-certified counterpart, the statute does not specify that the decision be made on a model year basis.
Vehicles manufactured for sale in the United States are typically assigned model year designations for marketing purposes. Although the model year traditionally begins on September 1, it can begin on other dates as well. A date that is more important from the agency's perspective is the vehicle's “date of manufacture,” defined as the date on which manufacturing operations are completed on a vehicle at its place of main assembly. See 49 CFR 571.7 and 49 CFR 567.4(g)(2). The agency uses a vehicle's date of manufacture to identify the specific FMVSS requirements that the vehicle must be certified to meet. Manufacturers of vehicles intended for sale in the U.S. must affix to those vehicles a label that, among other things, identifies the vehicle's date of manufacture and certifies that the vehicle complies with all applicable FMVSS in effect on that date. 49 U.S.C. 30115; 49 CFR 567.4(g). The model year designation that a manufacturer assigns to a U.S.-certified vehicle has no bearing on the vehicle's compliance with applicable FMVSS.
Many European manufacturers do not use a model year designation for vehicles manufactured for their own markets. Instead, they rely on the calendar year in which the vehicle is produced. Moreover, the countries in which these vehicles are produced generally do not assign model year designations. Although, as previously noted, September 1 through August 31 is commonly accepted as the model year for the purpose of marketing vehicles in the United States, these dates have limited relevance, if any, to vehicles that are produced for sale abroad. Consequently, the agency is proposing to amend 49 CFR 593.4 by deleting “the calendar year that begins on September 1 and ends on August 31 of the next calendar year,” as one of the alternative definitions of the term “model year,” and adopting in its place “the calendar year (i.e., January 1 through December 31) in which manufacturing operations are completed on the vehicle at its place of main assembly.” This language corresponds to 49 CFR 567.4(g)(2), which identifies how the date of manufacture is to be selected for the purpose of a vehicle's certification label.Federal Register