PARIS -- The most important event of France's presidential campaign occurs on Monday, two months before electioneering starts in earnest here. On Oct. 26, Francois Mitterrand turns 71.

Only his advanced age is likely to keep the incumbent French president from seeking, and winning, a second seven-year mandate. My sense is that after long hesitation Mitterrand has decided to run and the election will turn on the electorate's view of his ability to finish a new term.

He will not announce that decision until February. But as the signs of his candidacy multiply, his health and alertness will come under the kind of scrutiny that was applied to Ronald Reagan, another septuagenarian leader who won reelection, back in 1984.

Mitterrand does not seem to be burdened by his age in the way Reagan often does. He has just returned from barnstorming trips across South America and West Germany, where his vitality and agility astonished friend and foe alike. His perceived victory over what de Gaulle once called the "shipwreck of old age" helps him float effortlessly above his rivals in the public opinion polls.

But French voters are keenly aware that in the 29-year history of the Fifth Republic, none of their presidents has finished a second term. De Gaulle resigned, Pompidou died in office, Giscard d'Estaing was defeated. These are hardly propitious omens for Mitterrand.

Neither is the ragged ending that the Reagan presidency risks today. Mitterrand's advisers have long feared that his standing could suffer at home if the 76-year-old American leader were to "go ga-ga," as one aide put it months ago, at the end of his term.

As the Democrats found out against Reagan in 1984, age is not a successful attack issue. Only the mental lapses or visible frailty of the older candidate can make age into a swing factor. Reagan showed that even then it can be quickly overcome if handled well.

Mitterrand's conservative opponents appear to have come to the same conclusion. They publicly rattled the spectre of a doddering Mitterrand unable to rule if elected, but quickly dropped it when their comments drew more ridicule than support a few months ago.

In a televised interview, Mitterrand recalled that it was the conservatives who had failed to renew the license of a television channel that showed only rock music videos. He regretted this, he said, since he enjoyed rock music himself.

Known as the Florentine because of his ability to maneuver in the shadows, Mitterrand is at the top of his game in the ambiguous situation that the defeat of his Socialist Party in the National Assembly elections in 1986 has created. He has turned the party defeat into personal advantage by redefining the presidency into part super statesman, part national referee.

In this "cohabitation," many expected Mitterrand to be eclipsed quickly by Jacques Chirac, 54, the dynamic and resourceful conservative prime minister forced on Mitterand by the elections. The president's formal powers are limited by the constitution to oversight of foreign affairs and defense.

But Mitterrand has nimbly used those powers to rebuild his image. As he demonstrated again this week with a well-publicized state visit to Bonn, he has taken complete control of the vital French-German relationship, an important electoral asset here. He has been totally supportive of Chancellor Helmut Kohl while Chirac has angered Kohl and others in Bonn by voicing serious reservations about their arms control policies.

Two dominant factors are pulling Mitterrand into a new campaign despite his own desire to retire and write his memoirs. One is the collapse of the Socialists, who appear unable to win or even run a serious campaign if he is not the candidate.

Second is the president's growing personal animosity toward Chirac, who will run in the presidential election next spring. Their relations, once cordial, have now soured to the point that Mitterrand appears to friends to be committed to blocking Chirac's run for the presidency, whatever the cost.

Many expect Mitterrand to attempt to deal with the age issue by proposing shortening the presidential term to five years if he is reelected. My information is that he will leave this ambiguous in the campaign, just as he will cloak his plans for constructing a new parliamentary majority.

Ambiguity is to Mitterrand what the briar patch was to Br'er Rabbit. He is comfortable in it, and turns it to his advantage. It is a talent that clearly improves with age.