Dietary supplements in America are very loosely regulated. The law that governs this is the Dietary Health Supplements Act. Tl;dr DSHEA classified dietary supplements as a food or food product which the FDA classifies as "generally recognized as safe." (GRAS)

It also said that so long as a manufacturer is using a vitamin, mineral, or dietary substance that was available before 1994 then are ok to manufacturer and it would be considered safe (GRAS.)

There are additional requirements about manufacturing processes available before and after that date but that is the gist.

The problem with this law is that supplements are not required to undergo any kind of testing or validation. If a dietary supplement causes a problem, the FDA does not really have the authority to order a recall. Instead it has to depend on the Federal Trade Commission, which can order a recall and levy sanctions due to mislabeling.

Anyone can manufacture and sell a dietary supplements and sell them to anyone, there are no age restrictions. It's something that IMO needs greater oversight.

DSHEA: https://ods.od.nih.gov/About/DSHEA_Wording.aspx

General Recognized as Safe: https://ask.usda.gov/s/article/What-are-Generally-Recognized...

FTC and FDA: https://www.ftc.gov/news-events/news/speeches/ftc-dietary-su...

Congressional Research Service, 2021: "Regulation of Dietary Supplements" https://sgp.fas.org/crs/misc/R43062.pdf