As for the other reigning monarchies: is the legal age of adulthood also the minimum age for accession to the throne without a regent? If so, this would mean that most can be headed by sovereigns as young as 18 ... Did it actually used to be 21 in Belgium?
The reason for my asking is that King Baudouin couldn't immediately ascend the throne after the abdication in 1950 of his father, King Leopold III. He had to wait a year -- being formally enthroned in July, 1951 (two months before his 21st birthday). Evidently the law must have changed, since Crown Princess Elisabeth celebrated her legal majority in 2019: it had been made clear that at 18, she was eligible to reign without a regency.
Does anybody know when exactly 18 became the age of majority in Great Britain? Was it already the law of the land before the accession of Queen Victoria? Or did it used to be higher, but lowered specifically for her sake? Her uncle and predecessor on the throne, King William IV, prayed that he would live at least long enough to see his niece and successor become a legal adult -- thereby staving off the scheme of his hated former sister-in-law, the Duchess of Kent, to act as regent of the kingdom on behalf of her daughter. As it was, he barely made it.
The circumstances of the accession in 1848 of Emperor Franz Joseph of Austria-Hungary were vastly different: he succeeded a great-uncle who had abdicated the throne (amidst the wave of political revolutions that swept Europe in the year), bypassing a father who had renounced his succession rights.
As for the unfortunate King Ludwig II of Bavaria: I strongly doubt that the minimum age was LOWERED to 18 -- let alone specifically to enable him to succeed without a regency. It had to have been already the law of the land -- which I really think, in his particular case, should have been different: imagine how better things would have been, had 21 been the legal age.
For in that case, it's likely that his uncle Luitpold would have served as regent during the early years of his reign. The historical record has it that Ludwig was in tears over his father's death -- not because he mourned the loss of a parent, but because he dreaded the prospect of being king so young. As it was, he had no choice about the matter, forced onto the throne when not yet 19.