How can we combat ageism at work?

It’s wise not to draw attention to your age but do not stand for ageist behaviours; you probably have a code of conduct in your workplace, so contact HR in such situations. Make sure to never directly answer illegal questions.
Engage in the world, stay active, mentally and physically, focus on your experience and stay technically proficient. Focus on what you can do and what companies can use that skill.
Be as independent as you possibly can and surround yourself with active and understanding people. If you make yourself and your knowledge both up-to-date and valuable, you can combat age discrimination.