Mature student teachers say they are being "thrown on the scrapheap" by ageist employers, who fear they are either past it or too expensive.
The newly-formed Association of Teachers Against Ageism says more should be done to warn late entrants about job prospects.
"Ageism isn't as emotive as other 'isms', but it's still a totally irrational form of prejudice", said Steve Jackson, ATAA's founder. "There's national publicity about a teacher shortage, but older trainees need to be warned most employers don't want them."
The ATAA wants to see a code of practice to discourage schools from advertising jobs with upper-age limits. There is currently no law against these. The association also intends to lobby the Teacher Training Agency and training institutions to warn mature candidates of the possible employment pitfalls.
Mr Jackson said: "Ageism is endemic in British society. Successive governments have pushed through legislation to combat similar discrimination on the grounds of race, gender or disability, but ageism has been largely ignored. "
Mr Jackson retrained at 40, after leaving his job in the building industry following an accident. He says a variety of careers counsellors and university admissions tutors "fired him up" with enthusiam, and never told him finding work might be difficult. Seven years on and working as a supply teacher he says he "earns less than I would do if I sat at home and claimed benefits".
He added: "Supposedly mature trainees cost too much, but I have stated on application forms that I would accept the same salary grade as a 22-year-old, to no avail."
An NUT spokeswoman said: "For cash-strapped schools, taking on a mature teacher with a higher salary is an obvious deterrent. But that is the fault of the current salary structure and the financing of schools. The profession needs to attract mature entrants who can bring a wide range of experience into the school."
The association would like to hear from mature teachers unable to find work or on low pay scale. Contact Steve Jackson at 5 The Villas, Great Hatfield, East Yorkshire HU11 4UT.
Opinion, page 10