one explanation is that they have been exposed to environmental irritants like smoke and pollutants for long periods of time. another explanation (and the two are not necessarily contradictory) is that the respiratory tract infections common among the elderly make it easier for asthma to develop.
common asthma symptoms among young people ---wheezing and breathing difficulties during exercise --- are less common among the elderly, who tend to be less active physically. therefore, asthma among the elderly id often misdiagnos as emphysema.