Short answers, pulled from the story.
The biological definition of an adult is an organism that has reached sexual maturity and is therefore capable of reproduction. Many dictionaries phrase it as the stage of the life cycle of an animal after reproductive capacity has been attained. In animals broadly, an adult is one that has reached full growth.
The age of majority ranges internationally from 15 to 21, with 18 the most common. Nigeria, Mali, the Democratic Republic of Congo, and Cameroon set it at 15, while Mississippi, Puerto Rico, and Bahrain set it at 21. Parts of Canada, Nebraska, Alabama, and South Korea use 19.
No, the age 25 claim is incorrect and outdated. It came from a brain imaging study by Jay Giedd dating to 2004 or 2005 whose participants were only aged up to 21, with Giedd assuming maturation would finish by 25. More recent studies show prefrontal cortex maturation continuing well past age 30.
The US voting age was lowered to 18 by the 26th Amendment in July 1971. The change responded to young men between 18 and 21 being drafted to fight in the Vietnam War, captured by the slogan old enough to fight, old enough to vote.
Emerging adulthood is a distinct developmental stage between adolescence and adulthood occurring between ages 18 and 25, named by psychologist Jeffrey Arnett of Clark University in Massachusetts. Its five features are identity exploration, feeling in between, instability, self-focus, and having possibilities.
Jewish tradition reaches adulthood at age 13 for boys and 12 for girls, marked by the Bar or Bat Mitzvah. The young person is expected to show readiness by learning the Torah and other Jewish practices.
Several US laws keep 21 as a benchmark for activities carrying risk. The National Minimum Drinking Age Act of July 1984 required a drinking age of 21, the Gun Control Act of 1968 bars handgun purchases from licensed dealers under 21, and the federal tobacco and vaping purchase age was raised to 21.