AS Bill Clinton was the first baby boomer president, Barack Obama could be the first Generation X president.

Or, depending on how you figure it, Mr. Obama, born in 1961, could be the third boomer in chief, following Presidents Clinton and Bush. In theory, the candidate Obama belongs in the boom, defined by the Census Bureau as births during the years 1946 to 1964.

But the practice of defining generations is more complicated than the theory. Often their labels are about as helpful as parents who imagine that if two children are 7, they should play together.

Generations have a natural fluidity  it can be hard to say where one group ends and the next begins. And in the case of the current presidential campaign, some clashes between age groups are as contrived as the groups themselves.