To Own a 12.2 prohibited, you needed to own one on Dec 31, 1977. Meaning the minimum age to own one is 57 years old.
For 12.3 and 12.4 prohibs, you needed to own one on July 31 and July 27, 1992, respectively. Minimum age 43, depending on your birthday.
For 12.5, prohibs, cutoff was Jan 1 1995, minimum age 40 years old.
For all of the above, to own a given firearm at such a young age would have required a young person, on or near their 18th birthday to go out and purchase a typically rather expensive and rare firearm, and at full market value, not today's bargain prices. Alternatively, they could have been given such a firearm by a relative, or as a result of inheritance. In reality I suspect the majority of these firearms are in the hands of grandfathered individuals who are much older than the minimum, or are in the hands of ineligible heirs who kept them without any avenue of doing so legally.
For 12.6.1 handguns, there is no age cut off, as they can still be transferred under 12.7, however you needed to have at least one in your family tree December 1, 1998.
I am younger than 38 and I have legal prohibs. As soon as my kids turn 18 and are eligible for an RPAL, they can have one too if they want it. Fortunately my grandfather went on a shopping spree of pre-46 12.6.1 handguns before he died, to ensure all of his descendants could have a prohib if they wanted one.