The elderly population is going to be greater than anticipated by the end of the century.

Women will still be living much longer than men into the next century, and the difference in their life expectancy will grow despite the fact that more of them are being subjected to the same job stresses that men are.

The nation will see great shifts in the ratio of elderly parents to the number of offspring expected to support them, and by the year 2020 the number of elderly will be more than twice the number of people aged 45 to 49.

Babies born last year can expect to live 73.5 years,

These projections were contained in recent testimony by Jacob S. Siegel, the senior demographic statistician of the Census Bureau's population division, before the House Select Committees on Aging and on Population.

The Census Bureau's 1977 projection of the nation's population aged 65 and older in the year 2000 was 31.8 million, whereas two years earlier the projection was 30.6 million, Siegel said.

"Mortality has fallen a little more rapidly than we anticipated in 1975," he explained in an interview last week.

"If fertility moves up toward replacement level - that is 2.1 children per women - we can expect about 12 percent of the population to be aged 65 and over in the year 2000 and about 15.5 percent in 2020, as compared with 10.7 percent in 1976," he said.

Siegel told the committees that the possibility of "a higher proportion of elderly persons in the population and a higher ratio of aged persons to persons of the usual working ages, suggests that government may be expected to play a bigger part in the support of the elderly, particularly in providing health and other services."

Noting the projection that men will continue todie at earlier ages than women, Siegel testified that the "ultimate price in life dissatisfactions" is "immense and incalculable."

He cited the "large excess of women at older ages . . . major economic losses to families and society and loss of familial psychological and social support for the surviving women."

He referred to a table showing that average life expectancy for a child born in 1976 was 72.8 years. For males it was 69 years and for females it was 76.7 years. Among those who were 65 in 1976, males could expect to live 13.7 more years and females 18 more years.

The table projects that girls born in the year 2000 can expect to live 78.3 years and boys 70 years. Women who are 65 then can expect to live 19 additional years and men 14.2 more years. In the year 2050 newborn boys will probably live 71.8 years and girls, 81 years. Men aged 65 that year will probably live 15 more years and women 20.7 more years.

Siegel said the continued divegence in men's and women's lifespans "shows and inconsistency since more women are smoking and more are in the work force.

"Perhaps the explanation is that basically we don't have yet an equality of work stress," he suggested. "Men are still unliberated from the overwhelming stress of work or perish. While many women must work, many others still are in families with two incomes and wouldn't starve if they didn't work."

in his testimony he said the large difference in life expectancy "reflects both a biological advantage of women and the persistent, sizeable differentiation of the life styles, roles, and personal habits of the sexes."

Figures on the ratio of elderly (ages 65 to 79) to middle aged people (45 to 49) show that in 1976 there were 156 elderly for every 100 in the upper 40s; in 1980 the ratio is expected to be 180 to 100: in the year 2000 it is expected to drop to 125 to 100 and in 2020 it will jump to 216 to 10, Siegel said. The figures shift with fertility rates, he added.

"With declining or low fertility, he told the committees, "elderly persons will have fewer brothers and sisters and fewer children than in the past to provide needed ot desirable economic and psychological support."