You can view it not as punishment but as a restriction caused by inability to adequately estimate risks. Similar to prohibiting teenagers and kids from driving cars.

These are adults, not children. Quarantine measures applied to adults based on age are unprecedented and unjustified.

From a legal standpoint, not exactly. For instance, people in the US are prohibited from buying alcohol until 21yo. Do you consider that restriction also unprecedented and unjustified? :)

You've chosen a worthless example. The 21st Amendment gives states the authority to restrict alcohol sales. It is explicitly carved out as a legal exception and thus legally irrelevant to the issue at hand.

Why is it worthless and why does the 21st amendment matter? The 9th amendment specifically allows states to legally preside over things that the federal government is not, so these laws are directly comparable other than that the 21st amendment in this case just specifically forbids the federal government from overriding the states decisions using the commerce clause.

There is clearly legal precedent to age discrimination/restrictions in the United States, from age related employment discrimination protections not applying to those under the age of 40, to the legal age of alcohol and recreational marijuana consumption (the latter of which is not somehow "exempted" by the 21st amendments restriction of the federal government).