Ricardo Thornton Jr. is a year old. He turned 1 on Friday, and yesterday friends of his parents gathered in the living room of his parents' Northwest Washington town house, which was lovingly draped with "Happy Birthday" crepe paper. But they were celebrating much more than the passing of a year.

Little Ricky's parents, Donna and Ricardo Thornton, are mildly retarded. They have defied the odds. After surviving life in institutions, they married three years ago -- something still rare among mentally retarded persons and against the law in some states. They've struggled to live independently, and now they have Ricardo Jr.

"That was my dream," Donna Thornton, 35, said, pointing to her son as he crawled around the kitchen. "When I was at Forest Haven, I thought I would be there for life. I asked them, 'How do you get out?' They said, 'You have to work your way out. You have to climb that ladder.' That's what I did. I climbed my way out.

"I wanted to get married, be like everybody else," she added. "I wanted a man, a right man, one to help me out and marry me. I got him -- and I got my baby."

The Thorntons are classified as mildly retarded because their IQs are just below 70. An IQ of 100 is considered normal. Their son, to their delight, has shown no signs of retardation.

They were married in an elegant ceremony attended by many of their friends from Forest Haven, the city's institution for the mentally retarded, now under court order to close. Their wedding was recorded in a Washington Post article and they later appeared on CBS' "60 Minutes." Now there's talk of a movie.

Gone are the days of living in an institution with windows covered by bars. The Thorntons' two-bedroom town house is spacious. Little Ricky's room is filled with toys; a rocking chair, where Donna Thornton cradles her son, and a small black and white television set. Their home is monitored by a nonprofit group that assists the mentally retarded.

During the past year, the Thorntons have overcome challenges and problems that a growing number of mentally retarded, or developmentally disabled, people are facing, as the nation turns from a philosophy of institutionalization to one that embraces independent living.

"I think being a father is a challenge," said Ricardo Thornton, 28, who works as an aide at the Martin Luther King Jr. Library in downtown Washington. "He took me away from a lot of things I like to do, like going to school. He took Donna from working. I've learned a lot of new things, too, like how to put on Pampers," he said, laughing.

"All I can say to people who are going to be daddies is, it's a challenge. Are they ready to meet that challenge? I was."

"It hasn't been easy," said Shirley Rees, a community liaison for the Bureau of Community Services, a division of the District's human services department. "The hardest thing for Donna to learn was you can show love.

"I would make a point of picking up the baby and cuddling him so she could see me do it," Rees said. "She would watch me and imitate me. It hit me one day that here is a person who probably never was hugged as a child."

Marriages like the Thorntons' have sparked debate nationally over whether people with low IQs can be good parents. Opponents contend that the children of such couples may not learn to talk properly or may have difficulty in school because they don't receive adequate stimulation at home. Proponents say that society doesn't decide who should be parents based on their race or income, so IQ should not be a factor either.

Then, too, there is the issue of how much support states should be required to provide to such couples, along with questions about the cost of those services.

In many states, the rights of groups such as the mentally retarded and the permanently disabled are restricted, experts say. Even if marriage is allowed, some states take custody of all children from the marriage. In several instances, courts have removed children from the homes of mentally disabled parents.

But the District's laws do not restrict marriages between mentally retarded people. Instead, marriage is treated legally as a contract between two mentally competent persons. Mentally retarded couples can marry in Maryland and Virginia.

For the mentally retarded, marriage is still rare. But, experts say, it can be expected that a growing number of developmentally disabled people may choose to marry.

"It's a whole new ball game," said Reginald Wells, acting chief of the District's Bureau of Community Services. "It's a complex issue. It is difficult to know how much or how little to do to maximize a person's growth. When do we let go? It's questions like this that we will have to grapple with."

The District now trains group home operators in sex education so that they can answer questions and teach mentally retarded residents. Recently a group of parents of mentally retarded adolescents asked the city to counsel them, too.

Social workers are linking mentally retarded people with other people to provide role models for those who spent years in institutions. A couple of women who are not retarded bring their children over to Donna Thornton's on Saturdays so the children can play and the parents can talk. Recently, Ricky started going to a nursery school.

"The people at school say he's doing good," said his father. "We're going to school to meet his teachers and they're going to tell us what kind of work they'll be doing with him so we can keep it up at home."

"I miss him when he's gone," said Donna Thornton, who expects to go to work soon as a housekeeper at a hospital.

They are divided on whether they'll have another child, but even if they do, they imagine it will be at least five years from now. "Too expensive," said Donna Thornton. "Let me get on my feet first."

But yesterday, for at least three hours in the afternoon, time seemed to stand still, and none of the national issues mattered. At the party, there were friends from Forest Haven and from special-education classes, a couple of unwed, handicapped mothers they had met in parenthood classes and some children diagnosed as mentally retarded.

Donna Thornton made a lemon sheet cake. Ricardo Thornton bought his son a pair of corduroys and a matching shirt and spent a lot of time taking pictures. They noted that Little Ricky has four teeth at the top and two at the bottom, can say "Da-da" (his first words), spends hours crawling after his mother and is about to take his first steps.

"We were concerned he would have some retardation. So far they say he is right on track. That makes us happy," said his father.

Donna Thornton, surveying a pot full of hot dogs and her lemon cake, said, "This is one happy life. When I get old and gray and people say, 'Donna, how was your life?' I'll say, 'It was wonderful.' "