Atlanta ? There were more signs Wednesday that those hardest-hit by swine flu are the young.

U.S. health officials said the median age for confirmed hospital cases in the United States is 15. And in Mexico, new figures showed that almost half of the 42 confirmed swine flu deaths were of people 29 and younger.

Some experts have speculated that older people exposed to more flu strains in the past have built up greater immunity.

At a briefing on Wednesday, Dr. Richard Besser, acting director of the Centers for Disease Control, said older people’s immunity might be one possible explanation. Or he said it might just be that younger people have tended to get sick first in the outbreak.

During the regular winter flu season, elderly people and those with chronic health conditions are the most likely to be sent to a hospital.