Older teachers feel they are being pushed aside and treated unfairly, a teachers' conference is to be told.
A survey from the NASUWT union suggests one in five older teachers have faced discrimination because of their age.
Rather than having their experience valued, older teachers complain they are being "marginalised".
"Abuse on the grounds of age is as serious as all other forms of bullying," the union's general secretary Chris Keates said.
While other forms of workplace discrimination might be publicly condemned, the teachers' conference heard claims that prejudice against older workers can go unchallenged.
"This survey demonstrates that ageism is still not being taken seriously enough," says Ms Keates.
Only one in 10 schools had an anti-bullying policy which referred to the issue of age discrimination, the union's survey of 3,525 teachers found.
Delegates at the NASUWT conference in Birmingham heard that one in three older teachers believed that they have been made to feel "less capable" than younger colleagues.
And one in five have felt that their "professional capabilities had been marginalised or undermined due to their age".
Some older teachers who were surveyed last month expressed fears that they were seen by head teachers as being too expensive.
The conference heard calls for greater protection for older teachers as they approach retirement.
This included calls for older teachers to have fair access to professional development and promotion and a "career exit strategy" as they get closer to retiring.
Ms Keates, said: "It is unacceptable that those who have given long service to the profession are made to feel incapable, inadequate and inconsequential. They are a pool of talent which should be valued."
Article by BBC News