Can someone who is 18 years old legally sign a rental lease/agreement in Nebraska?

Acccording to this site: Nebraska Legislature (http://www.nebraskalegislature.gov/laws/statutes.php?statute=43-2101)

All persons under nineteen years of age are declared to be minors, but in case any person marries under the age of nineteen years, his or her minority ends. Upon becoming the age of majority, a person is considered an adult and acquires all rights and responsibilities granted or imposed by statute or common law, except that a person eighteen years of age or older and who is not a ward of the state may enter into a binding contract or lease of whatever kind or nature and shall be legally responsible therefor.

So yes - an 18 year old can sign a lease agreement in Nebraska.

Some states will treat you as an adult upon reaching 18, but not Nebraska as ebaines points out.