Anthony Kennedy Shriver was a junior at Georgetown University in 1987 when he came up with what he thought was a really cool idea: Why not start an organization that would pair kids then labeled “mentally retarded” with volunteer buddies in a one-on-one relationship? He appointed his roommate treasurer, and his best friends signed on as the buddies.

Laura Dickinson/The Tribune via AP

From left: Maria Shriver, the late Eunice Shriver, and Anthony Kennedy Shriver at a Best Buddies event in 2005.

By his senior year, the classmates had paired up with 52 young people.

Today, Shriver is 46, the term “mentally retarded” is considered a slur, and the group that operated out of his dorm room is Best Buddies International, a nonprofit that operates in every state and on every continent except Antarctica. On Saturday, hundreds of cyclists will take off from the JFK Library in Boston and ride 100 miles to Craigville Beach, near the Kennedy family compound in Hyannis Port in the 13th annual Best Buddies Challenge.

For Shriver, the roots of working with the intellectually challenged were planted in his childhood; in fact, in his backyard. The youngest son of Eunice Kennedy, who founded Special Olympics, and Sargent Shriver, who started the Peace Corps, Anthony Shriver grew up on a farm named Timberlawn in Rockville, Md.

It was at Timberlawn in 1965 that his mother started a summer camp for children with developmental challenges that presaged the Special Olympics, which she would start three years later. “Timberlawn was a huge property with a pool, a barn, horse-riding rink, basketball court, fields,” says Shriver. “Big buses would pull up in our driveway, and kids would pour out.”

Jason Nuttle for The Boston Globe

Anthony Shriver with his Best Buddy Jorge Morilla at his home in Miami.

‘I was relatively new to the community and wanted to take part in some fun event. After being there with the buddies and the families, I have looked forward to it every year since.’ - Tom Brady

Decades later, at age 80, Eunice Kennedy started a summer camp for those with intellectual disabilities, and would get in the pool at UMass-Boston to teach swimming lessons. Until she was 82, she rode tandem with a “buddy” as part of the Hyannis Port challenge festivities. Both she and her husband were on the Best Buddies board; Sargent Shriver was chairman until he contracted Alzheimer’s disease.

Anthony Shriver’s Aunt Rosemary was another major influence. In 1941, when she was 23, Rosemary Kennedy was lobotomized because her father, Joseph P. Kennedy Sr., thought it would cure her mood swings. But she was left incapacitated and spent the rest of her life in an institution.

Rosemary and Eunice had always been close, and Rosemary would stay with the Shrivers for a couple of months every year. When her nephew Anthony grew up and moved to Florida, she stayed with him and his family for a couple of months each of the 20 years before she died in 2005.

Jason Nuttle for The Boston Globe

Shriver playing a game of foosball with Morilla whom he met through Best Buddies.

In envisioning Best Buddies, Shriver says he thought that giving others the chance to have a close relationship with someone verbally and mentally challenged would be “a special experience that would enrich their souls, enrich their hearts.

“Growing up with Aunt Rosemary has been a great experience for me and for my kids,” he says.

Shriver’s parents led by example, not by edict, and didn’t pressure their children to go into public service: “They wanted us to do something meaningful that we were passionate about.”

His brother Mark is a senior vice president for Save the Children, Tim took over Special Olympics from his parents, both now deceased. Bobby, the former mayor of Santa Monica, Calif., founded a nonprofit to aid Africa and a company to raise money for The Global Fund to Fight AIDS, Tuberculosis, and Malaria.

Vincent DeWitt for the Boston Globe

Anthony Kennedy Shriver crosses the finish line of a Best Buddies Challenge on Cape Cod in 2008.

The brothers help with Best Buddies, as does sister Maria, whose team last year raised $450,000 in the Best Buddies Challenge that runs from Carmel to San Simeon, Calif. Her daughter, Katherine Schwarzenegger, recently joined the board.

Hyannis Port was the first Best Buddies Challenge ride; the California Challenge was started 10 years ago; Washington, D.C., three years ago, and a Miami ride will be launched next year. That first year, 50 cyclists raised $200,000. This Saturday, Hyannis Port will host 1,600 riders with a goal of $4.7 million.

The rides — which also feature 50- and 20-mile alternative routes, as well as a charity walk and run — have raised more than $100 million for the organization. In addition to the friendship program, Best Buddies has a jobs program that helps buddies find and keep jobs.

“There’s a huge vacuum once they get out of school,” says Shriver. “Getting that social outlet, getting them employment is a critical part of getting them independent.”

Laurence Levin

Shriver with his uncle, the late Senator Edward Kennedy, at a Best Buddies Challenge in Hyannis Port.

One of Shriver’s buddies is Jorge Morilla, 41. They met 17 years ago and have been a pair since. “He knows all my kids, has been to their sporting events, to our house,” says Shriver. “We go to the movies. It’s not like, ‘Hey, I have to go see Jorge this week.’ It’s just like he’s a friend of mine.”

Shriver helped find Morilla a job; he works at the Hyatt Regency in Miami Beach as a special projects houseman in maintenance and on the loading dock. He has done 12 Best Buddies Challenge rides.

Best Buddies, says Morilla in an e-mail, has been his “Express to Success,” earning him friends, employment, “and the freedom to go places and do things I couldn’t ever do before.” He calls Shriver “a great friend and motivator,” and lists several activities he’s enjoyed with the Shriver family, including attending Miami Dolphins games in “a suite on the Club Level.”

Miami?

When Shriver graduated from Georgetown, he decided to build Best Buddies on a state-by-state basis. The state of Florida gave him a grant and he committed for two years.

He never left. “I liked the weather,” says Shriver, who lives in Miami, where he met his wife, Alina. The couple has five children, including 22-year-old Teddy, who is in Peru for the Peace Corps — the “first Shriver grandkid to join,” says Shriver.

Jay Connor/File

Shriver with Tom Brady and Buddy Ambassador Chris Harrington. Brady has been honorary chair of the Best Buddies Challenge for five years.

Like their father, his own children are growing up with social activism. They have all been closely involved with Best Buddies, organizing school chapters and events. Carolina, 10, created a campaign and had classmates sign a pledge not to use “the R word,” as it is called by advocates who want to erase the term “mental retardation.”

Shriver’s youngest, Joey, is only 2½. “But he wears Best Buddies T-shirts all over the place,” he says, laughing.

A shrewd marketer, Shriver has brought top athletes into the Best Buddies fold. “Our culture is celebrity driven and celebrity obsessed,” he says. “It helps us a lot when we try to sign people up. I think there’s no one bigger in New England than Tom.”

That would be New England Patriots quarterback Tom Brady. In 2002, Shriver asked Brady to ride in the Hyannis Port challenge.

“I was relatively new to the community and wanted to take part in some fun event,” says Brady, who rode the 20-mile course with his sisters. “After being there with the buddies and the families, I have looked forward to it every year since.”

For five years, Brady has been honorary chair of the event, and has a longtime Best Buddy named Katie. The night before the Hyannis Port ride, the Tom Brady Football Challenge at Harvard Stadium will feature Brady quarterbacking both teams, which will be captained by two buddies, and include a mix of other buddies and Patriots players.

Boston runs in the Kennedy bloodline, along with square jaws and good teeth. Though long a Floridian, Shriver visits the Boston Best Buddies office throughout the year, and his family spends the summers at the Shriver home in Hyannis Port.

Seyfarth Shaw donates the Boston office space, employs four Best Buddies, and its cycling teams have raised more than $1 million for the cause. The law firm this year will be presented with Best Buddies’ highest accolade: the Spirit of Leadership Award.

At the end of the Hyannis Port ride, cyclists are rewarded with a clambake on Craigville Beach, with a rock band. The party used to be held on Senator Edward M. Kennedy’s property overlooking Nantucket Sound, and Uncle Teddy would address the crowd, but it outgrew the space.

Best Buddies has come a long way since its days in the dorm room, but Shriver isn’t content to let it be. “Next year, we’re adding a one-mile swim off Craigville Beach, and we’ll have a triathlon in 2013. The name of the game is you gotta keep stuff new and different.”

At least, that’s what his parents always told him.