I imagine this was a political compromise to let politicians advertise the availability of low cost insurance plans for low earners like young people in jobs without health insurance so they were not hit with the tax penalty that used to exist for not having health insurance.

Over 30 is likelier to be making more money and in jobs that do subsidize health insurance so they are likelier to buy it. And since the whole scheme is actually a mechanism to tax, you cannot let everyone opt out of the tax.