How can we avoid ageism?
Embrace the value of diversity.
"Ageism" is just another stereotypical label for a particular prejudice, bias, or bigotry. When people act from prejudice about a group they are committing one of the most common errors in statistics as well. The "Population" statistics NEVER apply to the Individual!
So, whether ageism, sexism, racism, or any other "-ism", the solution is to recognize everyone as individuals - capable of contributions or detractions - but needing an individual acceptance and knowledge to find out which it may be.
Physical characteristics may have specific detriments to some very particular tasks. But the entire individual will most likely have many other ways in which to contribute substantially to the community, and thereby YOUR well-being as well!
ACCEPT that we are all different, with commonalities as well. LINK with the commonalities and look for the ways that the "differences" are really advantages to the overall community/business/social group.