If you were born before 1954 your promotion prospects as a teacher are zero. No matter how good you are or how wide your experience, you are finished as soon as the number four replaces the number three as the first digit of your age. There are exceptions, but they are usually women who have had a career break to have children. Men, it would appear, have no opportunity to go further.

Why is there this apparent worship of the thirtysomethings? I can guarantee that if a senior post or posts in your school have been filled in the past two years, they will be occupied by people in this golden age bracket.

There is presumably some solid reasoning for this. But what is it? I can make some inspired guesses. General wisdom seems to believe that, once past 40, you have not and cannot have an original thought in your head. Perhaps experience makes you more pragmatic, but are pragmatism and innovative thought mutually exclusive? Since most headteachers are over 40, are they admitting they do not have any original thoughts and they rely on plagiarism?

I have met 25-year-olds with more set ideas than 40-year-olds. Innovation is an attitude of mind rather than an indication of youth.

It also appears that headteachers appoint teachers on the basis that they want them to move on. Most head of department and deputy head details have this phrase, or similar, tucked away somewhere: “we are seeking to appoint someone who has the potential for further promotion”.

I have always found this a curious arrangement - you appoint someone in the hope that you will have to go through the same procedure in two or three years time. Does ambition die at the passing of the 39th year? Are you then doomed to nothing more than plotting a comfortable course towards retirement?

Two recent conversations illustrate the different attitudes between commerce and education. A friend who is in charge of personnel for one of our leading companies was astounded when he realised that teachers had no chance of promotion at 40. His comment was that the idea was outmoded. His company had “long since ditched” the policy, realising that age and experience brought an “invaluable” maturity of decision-making. It was the exception rather than the rule when somebody under 40 was appointed to a senior management post. “We cannot afford to have staff who feel that there is no progression because of their age. Ability should be the criterion, not age,” he said.

A few weeks earlier I had been talking to a headteacher advertising for a deputy. The profile he drew up included an age of between 32 and 45. When I asked if he would actually appoint someone aged 45, he said he would give it consideration, but it was unlikely. In the end, only one applicant over 40 was invited for interview.

Nobody should be surprised that the deputy he appointed was aged 38. This particular headteacher genuinely believed that age was not an important criterion and yet, in practice, he did not even respect his own limit of 45.

Staffrooms over the years have been centres for dissent and dissatisfaction, usually led by the “old lags”. Why? Simply because they have nowhere to go, no way in which they can prove their worth. It is indeed an odd system that scrap heaps ambition at 40.

Tim Bromfield is a teacher in the Midlands, writing under a pseudonym