Retirement age is the age that a person is required to retire by law. Usually it is 65.
Is a company managing out an older employee a form of retirement age?
Is not hiring older employees a form of forced retirement?
There is no good reason for it.
One of arguments for a retirement age is that older people are keeping their jobs longer and stop younger people entering the workforce.
This is not true, it is an over simplistic way of looking at things.
An easy way of discounting the theory is to look what happened when women entered the workforce.
When women entered the workforce, the amount of people in the labour force increased, but so did demand for goods and services. Subsequently men were not forced out as a consequence.
The same thing happened when wars ended and vast numbers of soldiers came back from war to enter the workforce.