What's the average age kids usually get asthma?

Variable. It depends on many factors, for instance family history, history of RSV infection as an infant, prematurity, allergies, environment etc.
Before age 3. Most children with persistent asthma with allergic triggers will develop symptoms before age 3. 60% of kids less than 3 without food allergy, eczema or a family history of asthma will not have wheezing after age 6. Unfortunately, most kids with asthma will have symptoms before it is easy to check their lung function. There is no age limit or age requirement to have a diagnosis of asthma.