Benjamin Sims II of Arlington cuts his own grass, cooks three meals a day, drives himself and a neighbor to the grocery store every week and climbs a ladder to clean the gutters on his house.

"That's a sore subject," said Sims's son, Benjamin III, peering over the top of his glasses. "He's still cleaning those gutters, and he's not supposed to be on any ladders."

The discussion is on getting old, and Benjamin III knows something about it. He's 65. The father who still needs lectures about ladders turns 100 today.

In an age obsessed with health and exercise, the elder Sims is an enigmatic example of how to live longer. He smoked only once in his life, but he eats eggs and sausage daily. Work and rest, not oat bran and wheat germ, are what Sims says keep him going.

"The fastest growing segment of our population is the 85-and-over group, and their mortality rate is decreasing 2 percent each year," said Rebecca Elon, a geriatrician at George Washington University Medical Center.

About 3 million Americans were 85 or older in 1989, according to the Census Bureau, up from 2.3 million in 1980. In 1980, about 15,000 people were 100 or older. By last year, the number was 61,000.

Sims, the fourth of 13 children born and brought up on a Greene County, Va., farm, has spent a long lifetime doing for himself. A carpenter who worked for Kennedy Bros. construction company in Washington from 1910 until his retirement in 1963, Sims still does his own handyman chores and helps others do theirs.

"His main hobby has always been tinkering," his son said. Sims built the picket fence that boxes his front yard, and among his friends and old customers his knack at fixing things has earned him an almost legendary reputation.

"I got to know him in the beginning because I needed a little carpentry work, and he was a beautiful workman," said Sally Reston, a longtime acquaintance.

Reston said the only time Sims ever admitted that his age might be getting the best of him was just last winter.

The Restons' cabin near Hume, Va., had sprung a leak above the kitchen door, and Sims was reluctant to start repairing it because the extent of the water damage was unknown and might have involved complicated construction work. He wouldn't do it, Sims told Sally Reston and her husband, James, who was the longtime Washington bureau chief and columnist for the New York Times.

" 'But I'll take a chair and sit out there and tell you what to do,' " she recalls Sims offering.

His insistence on staying involved in the world around him is something that may help Sims keep going at his current pace.

"I think that mental attitude does have a lot to do with how engaged we stay with our community and with those around us," said Elon, the geriatrician.

Sims has made some concessions to age. He hasn't been deer hunting since he was 95, and he has cut his trips to the horse track in Charles Town, W.Va., to once every three or four months.

"That's about the only excitement I've allowed myself," Sims said. "I've never been to a bowling alley or a pool hall in my life, and even if racing costs a little, it's better than running around places like that."

Sims will celebrate his birthday this weekend with a party of about 100 friends and relatives. He still has one milestone to reach before he sets any family records for age: He had a cousin who lived to be 103.

Sims thinks his prescription of work and rest is more likely the reason for his healthy old age. "There's no more to it than I just worked all my life and got my perfect rest," he said. "I went to bed early and I got up early."