“First Solemn Communion” was actually invented after Pius X encouraged First Communion at the centuries-old canonical age - the age of reason. Churches didn’t want to give up what they had turned into a coming of age (First Communion), so despite complying with the pope’s wishes they just added a new “First” Communion celebration to keep up their tradition of adolescent First Communion. Other areas simply moved First Communion without moving Confirmation, thus screwing up the order of the sacraments. It had been customary to confirm whenever the bishop came through the parish, which would usually catch the children from ages 7-11 or so.
In the early Middle Ages, parents were required to have their children confirmed within the first few years of baptism (which occurred as soon as possible). Because of noncompliance, this eventually got extended to “by the age of reason.” But people continued to neglect the sacrament of Confirmation (they didn’t bother to do it until after the “deadline” if they did it at all), such that eventually, by the mid to late Middle Ages, people said, well, since we don’t confirm anyone until the age of reason or later, that must mean we shouldn’t confirm infants. So instead of “by the age of reason” the law turned into “at (or after) the age of reason.” At this point, the “batch” method was firmly in place whereby the bishop would confirm all those of age when he came to (or near) a town, resulting in the roughly 7-11 spread mentioned above. After that Confirmation, the custom grew up in France in the 1800s of delaying First Communion to adolescence as a rite of maturity. When Pius X reinforced that the best age for First Communion was the age of reason (not even mentioning Confirmation, which, it went without saying, should precede Communion), the French turned Confirmation into their coming of age (or else used the new First Solemn Communion). As you can see, the idea that Confirmation is a rite of maturity is no older than the 1800s, and even then it remained fairly localized until the 1970s, when that suddenly became the dominant idea of the sacrament.