http://www.who.int/hiv/pub/guidelines/arv2013/intro/keyterms/en/

"Age groups and populations

The following definitions for adults, adolescents, children and infants are used to ensure consistency within these consolidated guidelines, as well as with other WHO guidelines. It is recognized that other agencies may use different definitions.

An adult is a person older than 19 years of age unless national law defines a person as being an adult at an earlier age.

An adolescent is a person aged 10 to 19 years inclusive.

A child is a person 19 years or younger unless national law defines a person to be an adult at an earlier age. However, in these guidelines when a person falls into the 10 to 19 age category they are referred to as an adolescent (see adolescent definition).

An infant is a child younger than one year of age.

These guidelines define key populations to include both vulnerable and most-at-risk populations. They are important to the dynamics of HIV transmission in a given setting and are essential partners in an effective response to the epidemic. People living with HIV are considered a key population in all epidemic contexts.

These guidelines define most-at-risk populations as men who have sex with men, transgender people, people who inject drugs and sex workers. Most-at-risk populations are disproportionately affected by HIV in most, if not all, epidemic contexts.

Vulnerable populations are groups of people who are particularly vulnerable to HIV infection in certain situations or contexts, such as adolescents (particularly adolescent girls), orphans, street children, people in closed settings (such as prisons or detention centres), people with disabilities and migrant and mobile workers. Each country should define the specific populations that are particularly vulnerable and key to their epidemic and response based on the epidemiological and social context.

Serodiscordant couples are couples in which one partner is living with HIV and the other is HIV-negative. A couple refers to two people in an ongoing sexual relationship; each of these is referred to as a partner in the relationship. How individuals define their relationships varies considerably according to cultural and social context."

it is better to change the question into: at what age a person is classified as old...?

retirement is the best proxy for it; US and Canada using 65 year as the standard for retirement. In Indonesia, common Government Official retired at 58 year-old; teacher at 60, university professor between 65-70s.

The criteria is usually related to the common trend happen to the person with that age; physical and psychological deterioration.