In the USA, it was 21 until the 1970’s. It got changed as people pointed out that you could be drafted and forced to die for your country at that age, but you couldn’t help choose the people to make that decision. Given than 18 is the age when you legally become a legal adult, it was decided that 18 made a good age to gain the right to vote.

A major argument I’ve heard around it becoming younger is compulsory education. There is a fear, likely unjustified, that high school teachers would get a lot of control over part of the voting block since they would be able to tell students how to vote.