It's usually used as a slur, though. Not letting 3 year olds drive is changing behavior based on age, but sometimes you should be treating people differently based on some external attribute they cannot help. If most people a certain age do not meet the minimum requirements for the task (such as being able to see over the steering wheel and having a good judgement about rapidly changing situations), it is not "ageism" to say they cannot do something. Banning 3 year olds from driving is different than only hiring under-40 software developers.