Quibble with your question. De facto means , in fact.

The question you are asking is when did 18 became the age of majority in law. That would be de jure.

Most nonlegal conversation wouldn't use de jure though

Additional question, if I may. Are there accepted timeframes when the age of majority was raised and lowered in the Western Civilization? As I understand it, it changed little from the antiquity and generally varied from about 15 to just over 20. If it wasn't bound by period, was there a geographical split?

Its 20 (hatachi) in Japan. Because this works better for math.

Well I can tell you some information about ancient Rome from a legal viewpoint.

Adulthood is firstmost characterized by legal capacity, the capacity to act with legal consequences. Legal capacity assumes reasonable and independent manifestation of the will, reasonable disposition of your own legal relations and responsibility for your own actions. A man acquires it at a certain level of physical and mental development, which, according to Roman understanding, coincides with sexual maturity (pubertas). Since sexual maturity does not begin for all people and in all regions at the same time, puberty was originally established in boys on case to case basis, and was manifested by the ceremonial act of assuming the toga of manhood (toga virilis), and also involved taking off his bulla and a shave. It happened between the age of 14-16. For girls, on the contrary, establishment of sexual maturity was not required and the beginning of their legal capacity was the end of age 12 (of course, it was very limited since women were under a lifelong tutelage (tutela mulierum); however those limitations with time weakened and in the later Roman law remained only as a formality).

This was the old standpoint on establishing maturity, which even in the imperial period of Rome (roughly after Augustus in 27 BC) was advocated by one of the two famous schools of legal tought, the Sabinian school. The other one, the Proculians saw a need for a fixed age for maturity and determined it to be end of 14 for boys and 12 for girls. This later view was also partially adopted by Justinian in his Digestae (6th century).

Amongst the immature (impuberes) under that age are children (infantes) "who can not yet speak" (qui fari non possunt - meaning they lack the capability to pronounce old formalistic legal formulas), in other words, a complete lack of legal capability. Justinian designated the end of age 7 as the limit of infancy. That was the first period of becoming an adult in the eyes of the law.

The second period was immaturity above infancy (impuberes infantia maiores; qui fari possunt) and they had only limited legal capacity, they couldn't marry or make a will, and they could only enter into legal transactions that bring them benefit (such as gifting) and not transactions which include a reduction of their property, possessions or obligations. For those kinds of transactions they needed the participation of a tutor (tutoris auctoritas). The mature (>14;>12) were in the early days of the Roman state had full legal capability and if they weren't girls, their tutelage would stop.

This early onset of maturity, which also occurs in other ancient laws, proved to be unsuitable for developed and more complicated economic relations. Therefore lex Plaetoria (around 191 BC) introduced a new age limit and a third period which lasted until the age of 25 (minores viginti quinque annis). This law also provided a criminal and a popular (meaning everyone can file a lawsuit) lawsuit against those who would abuse the inexperience and naivety of a minor (14<25) and deceive them to enter into a damaging transaction for them. The praetor also provided more protection for minors through his edicts as well as the possibility of legal reinstatement (restitutio in integrum) with the effect of retroactive nullification of the legal transaction.

Because of those means of legal protection, minors did not enjoy the confidence of third parties in commerce. Therefore in the classical era of roman law (roughly around the era of the Principate (27 BC - 235 AD)) the magistrate would, on minors request, appoint them a curator for that specific transaction with whom the above mentioned means could not be as easily used. So starting from Marcus Aurelius (161 AD), a custom developed of appointing permanent curators for all minors transactions and in the post-classical era a permanent cura minorum was created for all persons under the age of 25. Finally, Justinian equalized cura and tutela and de facto moved the age of adulthood to 25 and the legal capability of minors was almost as that of impuberes infantia maiores except they could marry and make a will.

Also, in the post-classical era, a minor under the age of 25 could attain adulthood with the emperor's permission (venia aetatis) and would gain full capacity to conclude legal transactions (except alienation and hypothecation of immovable property) and would be freed from curatorship as well. In the later Empire (from Constantine) venia aetatis was granted only to men over 20 and women over 18.

I wrote this in a hurry so if you have any questions don't hesitate to ask

My understanding is that in the ancient world, until age 17 a youth was not considered to be strong enough to hold his place in a battle line. Reasonably that was the determinant for complete adulthood.