Is it common to develop asma at 42 years old?

Not common.: Not common, but possible. 75% of asthma is diagnosed in early childhood, but a number of people are not diagnosed until mid life.

Yes, with co-factors: The symptoms of asthma can develop after some factor causes bronchial tube inflammation. For example a severe lung infection, an allergic wheezy reaction, an exposure where you breathe in harsh chemicals or even the early signs of COPD after years of smoking. These can trigger lung damage through inflammation which will make you more likely to wheeze. Most treatments help control the wheezing.