Age 18: An adult can vote, drive, fight for our country, serve on a jury, get married, sign legal documents, buy cigarettes, go to jail, and adopt a child.
These things indicate maturity. If an 18 year old is mature enough to do these things then they are mature enough to consume beer or wine.
In addition, the current drinking age encourages under-age college students to drink alcohol in unsafe environments. It encourages rebellious behavior that leads to accidents and trouble.