If the life expectancy of a child born with hiv is 3 years, then why is it the people catch hiv as an adult? how did it progress to adults having it?

Adult's 1st victims : This newly mutated virus was first recognized in sexually active adults. Infants were/are collateral damage and acquired the disease from their infected mothers. Today a well controlled HIV + mother has a great chance of having a healthy HIV - infant.

HIV: Uninfected humans can be infected for the first time at any age. E.G. One who is free of HIV infection can acquire an infection at any age.

?what?: Hiv is an infection and any person of any age can contract it. Children are often the unfortunate group who contract it from an infected adult who gives birth to them.

Depends where u live: In developed countries like the usa medical science has developed new drugs to keep an HIV patient alive for a very long time. That medication is quite expensive ; if you live in a third world country this kind of therapy is not readily available. You also have to have an educated public because the regimen in taking the medication is very important.