What is a common risk factor for developing asthma in young children?

Exposure to second-hand smoke is a well-documented risk factor for developing asthma in young children. Research has shown that children who are exposed to tobacco smoke, whether in utero or after birth, have an increased likelihood of developing asthma and respiratory issues. This exposure can cause inflammation and sensitivity in the airways, making them more reactive to various triggers, which can lead to the development of asthma symptoms.

While other factors, such as altitude and socioeconomic status, can influence overall health, they do not carry the same direct and proven relationship to asthma development as exposure to second-hand smoke. For instance, living at low altitudes is generally not associated with an increased risk of asthma. Additionally, being over six years old typically indicates that a child is past the early developmental stages where asthma is frequently diagnosed, and high socioeconomic status can be linked to better access to healthcare and resources, possibly mitigating asthma risk. Hence, the significant evidence linking second-hand smoke to asthma makes it the most relevant option among the choices given.