Reply #41: Because, clearly,

[GirlinContempt]

since we HAVE to pick an arbitrary number for adulthood, we do. It works because we have to have SOME sort of line of definition that is functional. It ceases to be so when you change numbers for certain things. Part of what we DO is create laws that protect the most people they possibly can, and then deal with aberrations as they come, it's the best way humans have found to deal with regulating millions of people at a time. When you start saying "You're an adult at x age, but you can't do this until x age" it casts the whole system into question, especially about BIGGER issues that have more far reaching consequences.
If you think EVERYTHING should be upped to 25, I'd question your judgment, but that isn't the same as trying to pick and choose things that deserve a different age limit beyond the standard adult one.