From Wikipedia, the under-40 rule is based on Fields' desire that
"while it was in recognition of work already done, it was at the same time intended to be an encouragement for further achievement on the part of the recipients and a stimulus to renewed effort on the part of others."
From this statement itself, I cannot see any intention that Fields only wants to award the under-40 mathematicians. We can certainly award a 50-year-old mathematician as "an encouragement for further achievement".
In modern academic hiring postings, it is very common to see (at least widely accepted in the U.S.) the statements like
XXX is an Affirmative Action/Equal Opportunity Employer. We consider qualified applicants for employment without regard to race, religion, color, national origin, ancestry, age, sex, gender, gender identity, gender expression, sexual orientation, genetic information, medical condition, disability, marital status, or protected veteran status.
Therefore, it seems to be unreasonable from both historical point of view and modern standards to put such an age bar on such an arguably "most important prize in math".
Has under-40 rule of Fields Medal ever been seriously discussed by ICM (I mean seriously by IMU/ICM committee, not just the casual discussion by a small group of mathematicians), voted on to change or at least challenged?