Asthma can occur at any age. Most childhood asthma begins before the age of 5 or 6 years. Sometimes asthma results from exposure to a new environmental factor such as a new animal in the house. Asthma may begin as an upper respiratory tract infection ("cold") or as a chest infection ("bronchiolitis").
Although milder childhood asthma often resolves during the early teenage years, symptoms may continue throughout life or, in some cases, resolve during adolescence only to recur later in adult life. On the other hand, the first appearance of asthma may occur as an adult ("adult onset asthma"). This often occurs between the ages of 40 - 50 without any specific allergy being detected. The worst cases of lifelong asthma often begin before 6 months of age. However many cases of adult-onset asthma can also be quite severe.