SAN FRANCISCO (AP) - Supporters of an initiative to legalize recreational use of marijuana in California say the measure will make it harder for people under 21 to obtain the drug. California Lt. Gov. Gavin Newson, an early and prominent backer of the Adult Use of Marijuana Act, says the initiative includes tight licensing, labeling and advertising restrictions to keep anyone under 21 from buying marijuana. It was among several arguments in favor of legalization delivered by backers of the proposed initiative as they announced Wednesday they had collected 600,000 signatures of registered voters to qualify the measure for the November ballot. They had until July 5 to collect a total of 365,880 signatures. Supporters also said the state will benefit from taxing pot if the ballot measure is approved. Medical marijuana is already legal in California, which produces more pot than any other state.