Actually, that's not the case with the larger rental car companies, such as Hertz and
Avis. They are self-insured; they use insurance companies only for administration and for secondary coverage. So it isn't a matter that insurance companies are unwilling to underwrite the rental car companies, but rather that it doesn't make financial sense for the rental car companies to underwrite that risk themselves.
The Taft-Hartley Act pertains to labor relations -- it comes up most often here on the DIS when discussing potential airline strikes and other threats to transportation to WDW.
The federal government does tend to avoid entering into businesses that would directly compete with private
enterprise, but there is no actual law to that effect. There are actually many examples of where the government does compete, and in such cases they do so because there is a compelling public interest, but of course that's a matter of opinion, and some folks would hold the opposing perspective in each case.
However, that's also not really relevant to this situation. Rather, the issue here is with regard to whether government should regulate whom business can and cannot discriminate against in the offering of public accommodations. The answer is, however, a resounding YES. The government can and should regulate such discrimination. Having said that, there is no reason to think the government will or should prohibit discrimination against youth. While there is a significant personal interest, on the part of youth, to prohibit such discrimination, there is no compelling public interest in that regard, as far as I can see. To some extent, the opposite is true: The vast majority of the general public benefits from that discrimination, and there is no evidence that that discrimination fosters a culture of inequity, since everyone is equally subject to membership in that class, and all those who are discriminated against eventually grow out of the situation.