Thirty-nine, that magic age teetering between youth and maturity, will be the most typical age for Americans early in the next century, according to Census Bureau projections.

Historically a nation of youthful orientation, the United States is now thirty-something, heading upward, the Bureau reports.Combining to increase the median age of the population are the maturing of the massive post-World War II baby boom and that generation's relatively low birth rates, which reduce the number of babies to pull down the numbers.

So if present trends continue, census officials expect that half of all Americans will be over the age of 39 by the year 2010, and half will be younger. The agency's most recent estimate of median age was 31.7, as of 1986.

Whether America will plateau at 39 - as did the late comedian Jack Benny - remains to be seen.

The median age of the country touched 30 for the first time in 1950, then slipped back as the baby boom produced a bouncing crop of youngsters to lower the numbers.

It took until the 1980 census to hit 30 again, edged up to 31.7 as of 1986 and is expected to climb to 33 by 1990, 36.5 by 2000 and 39 by the year 2010, according to the new Census Bureau projections.

Utah will still be the youngest state. The state's median age will rise from 25.5 to 27.6.

Florida will remain the state with the highest median age.