Since professional firms are usually partnerships, the well-compensated people at the top of the pyramid are partners, not employees. As a result, all of that stuff that protects employees from discrimination on the basis of age in the Human Rights Code does not apply to them, at least where I live in Ontario, Canada.
With that over-simplified but pithy summary of the law, I have explained how partners in accounting firms and law firms can be squeezed out in their early sixties.