When the World Health Organization talks about older persons, they're talking about 65. I don't think anybody can give you an exact number. The immune system starts to lose efficiency at a certain point, too. But the rate at which that happens differs from person to person. Acheson would agree.
Different people lose their ability to fight infection at different rates.
It's an unpredictable decline. That knowledge can help people to make prudent decisions. Simin Meydani, a professor of nutrition and immunology at Tufts, takes a more hard-line stance, saying that age 65 is reasonable as a starting point for using increased caution concerning the foods you choose to eat. But no one can give you odds. For somebody who's 75, that's a greater risk.