Ronald Reagan is letting his hair grow gray. Jerry Brown says, only half facetiously, that "there's no substitute for experience."

If the leading Republican candidate for president and the young Democratic governor are right, then the man to watch in California is Evelle Younger, the state attorney general who is opposing Brown's bid for reelection.

For Younger truly personifies the qualities of mature competence that the better-known candidates are now trying to affect. I called on Younger in his Los Angeles office the other day and found a tall, pleasant, basset hound-looking sort of man. He checked his watch as I entered and remarked that I was right on time. "I've met a lot of important people since I became the candidate for governor," he said. "I find important people are always on time."

I let that piece of calculated unpretentiousness, not to say gross flattery, go by, and asked how the gubernatorial election shaped up, Younger said:

"In 1974 and 1976 Watergate dominated everything. People just wanted to get rid of the ins. So they voted for amateurs like Jimmy Carter and Jerry Brown.

"That's all changed now. People want somebody of demonstrated capacity. They choose a governor or a president as they choose a doctor or a lawyer or an architect. They want somebody who can do the job.

"That's why I think they'll choose me. I've always been one thing and they know what I am - a dedicated, career public servant. If buzz words make the difference, he'll win. If people take their stands on the issues, I'll win."

At that point Younger stopped to take a phone call, and I surveyed his office. It bore in pictures and various other memorabilia the marks of three decades of public office as city attorney in Pasadena, district attorney in Los Angeles and attorney general of California. There was also an old University of Nebraska football helmet, which I mentioned when our chat resumed.

"I was the slowest fullback in Nebraska history," Younger remarked. Then, as if concerned he might be offending somebody, he added: "Of course, there were a lot of fast fullbacks at Nebraska."

I returned to politics and asked why he was so sure he would win if people voted on the issues. He pointed out that Brown was against the death penalty, which the people of California had mandated in a vote for a constitutional amendment. He cited agricultural labor, where the people of California had defeated a proposition, supported by Brown, that gave more power to Cesar Chavez of the United Farm Workers.

He mentioned nuclear power, which the people of California had approved in a referendum but which, he said, the Brown administration had regulated to death. Finally, he cited Proposition 13, the initiative cutting property taxes, which passed overwhelmingly in the June 6 primary despite initial opposition from the governor.

"Brown now acts as though he's the author of Proposition 13," Younger said. "But he's got an integrity problem. Why did he change sides in the middle of the game? Why did he keep changing his position on the size of the available surplus?"

I asked Younger is he thought the presence of black candidates for lieutenant governor and attorney general on the ticket with Brown would hurt the governor. "You'd know better about that than I would," Younger said, backing away from the delicate subject of race.

Then he picked up again. "I tend to take the non-hysterical view of politics," he said with about as much hysteria as a man going off to sleep. "In elections people look at a general pattern and a track record. On every significant issue the people disagree with Brown. I think they'll vote on the issues, and that's why I decided to run against him. The question is whether they'll vote on the issues or for personality."

It is indeed. Brown is a political star who exudes body heat. He has come around on Proposition 13 with great aplomb. He now uses the issues to show that he is a strong leader who knows how to cope. He remains, for me, the most interesting figure in American politics. But I think Younger poses a serious threat to his race for the stateHouse this year, and to his hopes for the presidency later on.