And another thing I didn't know earlier...
In last week's TES, he said previous experience outside the classroom is necessary if a teacher is going to develop creativity and intellectual curiosity.
"Philip Pullman, who do you think you are?" said 28-year-old Markette, from Brighouse, west Yorkshire. "Pupils need teachers with enthusiasm, verve and passion. I feel fully experienced, thank you very much."
However, some contributors commended Mr Pullman for making a valid point, but suggested his selection of 35 as an appropriate cut-off age was somewhat arbitrary.
David, from Stafford, said: "By the age of 31 I'd fought in a war, gone back to uni and had walked on the wild side. I don't expect some wishy-washy writer to preach to me about life experience."
Others offered amendments of their own. "Until you are over 35, you don't realise how much you didn't know earlier. How about not being allowed to be a teacher until you've had a family?" said Vanessa, from Cowes on the Isle of Wight.
Away from the online discussion board there were practical concerns, which echoed across the profession. Steve Lesik, 52-year-old deputy head at Chesterton community college, in Cambridge, went into teaching at the age of 21.
"I didn't feel there was anything lacking when I went into teaching. Some of the best preparation I had was working in an abattoir, where I met a variety of people," he said.
"But I don't think we have any choice at the moment, because of the recruitment crisis. Philip Pullman doesn't appreciate the pressure we're under."
Lois Mills, a 25-year-old newly-qualified English teacher, at Dowdales school, in Cumbria, said: "Hell, if we all went into teaching at 35, we wouldn't have a pension to fund ourselves. We'd be working until we were 90."