Half the government's top paid, most important career executives have been around since Harry Truman walked and Dwight D. Eisenhower putted around Washington.

Most of the federal officials started at lower level jobs, going to night school and not reaching supergrade (Grade 16, 17 or 18) ranks until the Johnson or Nixon years. Most of those top aides today are older than President Carter. Many have or could have children who would consider key White House aides like Jack Watson, Jim King or Jody Powell to be youngsters.

Despite the heavy ration of career civil servants who started their government careers in the late 1940s and early 1950s, nearly 65 per cent of the political and noncareer executives in government today came in under Presidents Nixon or Ford. Thing about the very interesting mix.

A new study of the status and mobility of top political and career federal executives by the Civil Service Commission indicates that there is relatively little movement from the political arm of government to protected merit system jobs when administrations change.

Most people assume that whenever the Democrats turn over the White House to Republicans (or the other way around) there is a wholesale burrowing into the civil service system by political aides trying to protect their jobs and salaries.

The CSC study says there is less than a 1 per cent conversion rate, and that most of the changes are made by persons who have career civil service status, who take political jobs and then converted - quite legally - back to career jobs when administrations change.

Like most books of statistics, the new CSC study has something for everybody. If you take parts of it, you can prove that the executive service is relatively a politicial in a partisan sense, with job advancement being geared to skill, timing, luck and personal politics that govern upward mobility in any big outfit.

It can also be sued to show that each succeeding political administration - beginning with Eisenhower, and accelerating through Johnson and Nixon - has moved to politize the top levels of government management, adding more noncareer jobs. Congress also has helped by creating hundreds of new scientific and technical positions which the administration in power can fill with top nuclear physicists, or with out-of-work politicians.

During the Truman years, the study shows, 27 per cent of the people who now hold federal supergrade jobs (paying from $39,629 to $47,500) joined the government. During the eight years that Eisenhower was in the White House, 26 per cent of the present supergrade work force took the oath of office.In both administrations, the majority of the now-supergraders came in as relatively low-level workers.

Just over 20 per cent of the current supergrade federal career group joined the government during the New Frontier or Great Society periods. Nearly 15 per cent of the present noncareer federal executive population also came to work under Kennedy and Johnson.

Under Nixona nd Ford (mainly under Nixon) 13.9 per cent of the people who now hold Grade 16, 17 and 18 career federal jobs joined government, although many came in at leser grades. But substantial 65 per cent of the political or non career office holders in government today entered during that eight-year period.

Not all the Nixon-Ford era appointees are, of course, Republicans since the total includes many scientists and engineers, lawyers and others. But a lot of them are Republicans, or are believed to be, a point that will not be lost on the Carter White House, which is under fire from patronage-hungry Democrats and election aides to help them with their unemployment problems.

All the above leads some people to wonder why CSC made the study, which, as one official said, "servers up a fat pitch," in baseball parlance, to White House aides who are seeking a wholesale housecleaning of the top bureaucratic ranks.

Fall (Quitely) On Your Sword: Indications are that new Carter appointees will be asking many top career civil servants - the "lame, halt and Republicans" in their view - to hand in their resignations. Technically the career people can't be canned without much fuss and paperwork.

But insiders say the White House is hoping that senior career people won't kick up a rackus and will quetly fade away, carrying letters of commendation which they were given at the same time they were told to clean out their desks.