AUGUSTA - Maine is tightening up its regulations on purchases of guns by youths.

Gov. John Baldacci on Tuesday signed a bill that prohibits sales of firearms to 16- and 17-year-olds.

The law will allow exceptions for sales by a parent, foster parent or guardian - or if the young buyer gets permission from one of those. Violations can lead to a maximum fine of $500.

Most bills become law three months after the legislative session ends. It means the effective date could be in July.