Virginia today joins the growing number of states that have raised their legal drinking age to 21.

That law probably will have the most immediate impact of any of the hundreds of pieces of legislation from the 1985 General Assembly that became law today. But legislators and local officials say there are scores of measures that also will have widespread implications for state residents.

Other laws effective today will offer handicapped persons an agency to fight housing and employment discrimination, strengthen protections against sexual abuse and child exploitation, change the age at which children may enter kindergarten, expand voter registration hours and give modest increases to the state's welfare and unemployment programs.

Some of the new laws will produce immediate and visible results. Police throughout the state will begin a two-year project of changing all the flashing lights on their squad cars from red to blue. The reason: blue is considerably more visible during the day.

In Northern Virginia, effects of some of the most significant laws will not be readily apparent to residents.

"The laws we're happiest about are not going to be visible right away," said Mike Long, a Richmond lobbyist for the Fairfax County government. He cited what the county considered its biggest political coup of the 1985 session, a change in state highway funding formulas that should give more money to developing counties and a law that allows the county to spend an unlimited amount of local money on road construction and repairs.

"We're prepared to deal with all the changes from [requiring] public hearings for school board members to the cap coming off the road funds," said Fairfax County Executive J. Hamilton Lambert. "It's not like we're being hit cold. The staff has been briefed in their particular areas where there are changes in the law."

Fairfax County officials already have begun putting together plans for a multimillion-dollar trash-burning plant that would generate electricity. The legislature supported the facility after complaints from county officials that the suburban jurisdiction is rapidly running out of space to dump its garbage. The design of the $225 million facility, similar to a joint one being built by Arlington and Alexandria, must be approved by Fairfax supervisors.

Other new laws officially recognize premarital agreements, allow farmers to kill dogs attacking their livestock, allow blind persons to fish without buying state licenses and make the fraudulent use of a credit card number a crime.

The new drinking law makes it illegal for persons under 21 to buy, possess or drink beer. The law does, however, exempt those persons who already have reached the state's old drinking age of 19. The state law for wine and liquor remains 21 for all persons.

The increase in the age limit was seen as part of a national trend to raise the legal age in states to stem traffic fatalities and injuries of teenagers. The federal government has threatened to withhold federal highway funds from states that do not increase the age limit by 1986.

Maryland raised its drinking age for all alcoholic beverages to 21 in 1982. In the District of Columbia, the age is 18 for beer or wine and 21 for liquor, producing fears by Virginia officials that suburban teen-agers will flood into Washington to drink.

Alcoholic Beverage Control officials said they have begun distributing a series of brochures and posters, as well as radio and television public service announcements, to promote the new law.

Under the new Disabilities Act, a major initiative by Gov. Charles S. Robb, the state government is consolidating several agencies under a new Department for the Rights of Disabled. This should "provide a whole new series of protections to Virginia's more than 750,000 disabled persons," said Philip Abraham, Robb's special assistant for policy.

The law, initially opposed by influential business interests and state Chamber of Commerce, is a compromise measure, which is supposed to encourage employment opportunities for the physically and mentally handicapped and bar discrimination by employers when the handicap has no relation to a job. In addition, state services for handicapped persons now will be under one umbrella agency.

In the field of education, the state will require that children reach their fifth birthday on or before Sept. 30 to enter kindergarten. Children born as late as Dec. 30, the old kindergarten admission date, may be accepted if they pass an aptitude test.

Another new statute makes it a capital offense to abduct and kill a child 12 or under. Day-care providers will be required to make police checks of their employes in an effort to prevent the hiring of persons convicted of either child or sexual abuse.

In another major change for the state government, the state's approximately 80,000 employes will receive pay raises from 6 percent to more than 10 percent, with some guards in the problem-plagued corrections system receiving even more.

At the same time, about 450 top managers of state government, including prison wardens, will be removed from some civil service protections.

Robb sought the change after administration officials complained that policy changes in corrections, mental health and other agencies have been stymied by a cumbersome grievance procedure that made it difficult to discharge ineffective employes.