Germany

In fact, I find any argument that relies on the "maturity" or "capacity" of the voter extremely suspicious. There are people even today that genuinely believe that franchise must be restricted by means of literacy/mental capacity tests (transparent attempts to disenfranchise political opponents, be it conservatives fearing a more progressive youth or progressives fearing a more conservative older population).

The argument of relevance/degree of exposure to political decisions is indeed much better, than even going doing the maturity rabbit hole and trying to argue for a lower voter age that way.