Quoting your own link: > [a] form of argument that attempts to establish a claim by showing that the opposite scenario would lead to absurdity or contradiction.

The claim parent is attempting to establish is "how old a law is doesn't matter". The extreme example is ignoring an old law that abolishes the slave trade, and claims it is unimportant because it is decades old, which (parent is claiming) is absurd. Therefore, the original claim is absurd.