The lines around the edge of the mouth are a little deeper now, and some of the one-line jokes are older.

But Ronald Wilson Reagan, undeclared candidate for the Republican presidential nomination, showed few visible signs of his 68 years as he campaigned cross-continent last week on a tour that included six speeches, five interviews, two news conferences and various private meetings.

While Reagan won't formally announce his candidacy until this fall, probably in October, he is operating on a schedule that would be the envy of many declared candidates.

Reagan will have made 15 speeches this month and is to make another 17 in June, many of them for multithousand-dollar fees. He has appeared before GOP conventions in every region of the country, usually to overflow crowds of contributors.

In polls taken throughout the nation he has widened his already substantial leads over other Republican candidates, both announced and prospective. His only worry, if there is one, would seem to be that everything is going too well.

"It's a little frightening," Reagan said last week on the Reno to Las Vegas leg of a varied trip that included a speech to a forum of Canadian businessmen, appearances before Republican groups in Connecticut and attendance at the GOP's "Salute to Rockefeller" fund-raiser in Washington.

"I said to Nancy, maybe I've played on too many losing football teams. I feel a little exposed. You're out there riding point and everybody's aiming at you. There is something to be said for coming from behind when nobody is paying too much attention and you're driving to come up. And yet, you can't ignore the advantage of it. The advantage is there, and I'm just going to have to get used on it."

The advantages were evident last week, as Reagan consistently was given front-runner's courtesies that included a police escort at every stop. In Connecticut, where he had not appeared at a political event in years, moderate Republican activists queued up to be photographed with him.

On June 11, a 600-member Reagan committee spanning is to be announced in Connecticut, where a potentially important GOP primary will be held in March.

The attraction of this moderate support, both Reagan and his advisers say, reflects a growing conviction among a number of Republicans that the former California governor, like him or leave him, is destined to be the party nominee.

The one cloud on Reagan's horizon, as he gives his familiar speech celebrating the virtues of the marketplace, is a lingering belief in some party circles that voters will regard him as too old. Reagan's supporters have sought to defuse this concern in a variety of ways, the most direct being the introduction given Reagan at a $100-a-plate Hartford, Conn., fundraiser by John Davis Lodge, 75 former governor of Connecticut.

Lodge ticked off the names of world figures who had achieved great things at an advanced age, among them Winston Churchill, Douglas MacArthur, Konrad Adenauer and Ayatollah Ruhollah Khomeini.

"Ronald makes no such claim, preferring to rest his case on the more limited point that people will make their judgment on his actions and appearance rather than the year of his birth.

"I think people are going to make the decision on the basis of, do you look physically competent, do you seem to have all your faculties, and I think I do," Reagan said. "I don't think it's a major issue."

Major or not, Reagan appears to have made a special effort to show his crispness and competnece. If he makes a verbal slip, and he made few of them last week, Reagan takes pains to correct it, something he rarely bothered to do when he was running for governor of California.

Reagan has been campaigning in his unofficial way for the past several months. He plans to continue his schedule through June and July, and then take August as a month of vacation before a fall announcement.

While Reagan says he is flexible on the timing of an announcement, neither he nor his principal advisers see any reason to rush it. Reagan points out that he would have to give up his five-times-a-week radio program on 300 stations and his twice-a-week column in 100 newspapers if he became a declared candidate.

He says, also, that he would not be able to appear to many of the nonpartisan forums where he now makes speeches and would wind up talking to "those people who already are for me."

Except in a Toronto appearances, where his references to such American entitites as the Occupational Safety and Health Administration appeared to baffle his Canadian audience, Reagan won standing ovations and repeated applause at every speech last week.

On the plane, he read newspapers and wrote his radio broadcasts. The only concession to his age, and it may have been as much to the earliness of the political season, was that Reagan usually slept in each morning before beginning a full day of campaigning.

For an aging political warrior who sees himself as next year's nominee, Reagan's most positive moment probably came during a Canadian radio interview, where author Gordon Sinclair started to read from a magazine piece claiming that Reagan was too old to be president.

Midway through his reading, Sinclair snorted and said to Reagan, "I don't think you're too old."

Sinclair is 79.