It seems to matter much more in children's / YA writing. This is borne out by conversations yesterday with children's agents.
The Robe of Skulls (Tales from the Five Kingdoms)series.
But, importantly for this point, Viv and other writers "of a certain age" have been doing this for years and know exactly what they're doing. They keep very up-to-date with what's happening in children's writing. Not because they should but because they love it and respect it.
However, too many older writers, especially debut ones, simply don't. You would not believe the number of people who retire and think they'd like to write some cute stories for their grandchildren in their retirement. They think it's so easy - after all, how hard can it be to replicate the little stories of one's youth? Very hard, is how. And if you think of them as little stories you're already lost. You would not believe the derivative crud that too many would-be writers come up with. I have seen it and wept.
This is not about physical age but voice, and just as actors may have voice-training, authors need it, too. Especially children's authors. And voice takes practice and is harder to achieve for the first time if you're older, trying to write for children. It's by no means impossible, but it is harder. The only way to do it is to read the right books voraciously and analytically. And to practise. Vivian French has been practising for years. And writers like her and Joan Lingard (another friend) are trusted because they've got the voice right and know what they're doing. There are many, many others, some of them friends of mine, but I'm not even going to mention them here in case they don't want to talk about age... The point is, they have earned their right to be published by years of practice and honing that voice and keeping up-to-date with what is required of modern writers.
But for those of you who haven't been published yet and are trying to write for children and teenagers, please note: anyone can be a modern writer, whatever one's age. But are you a modern writer or are you mired in the past? Because as an older writer you will have to try extra hard to prove that you're not stuck in the past, because - and here's where the prejudice comes - agents and publishers will suspect that you are, if they know that you are an older, unpublished writer aspiring to write for children. This may be unfair, and you may be wonderfully in tune with modern requirements, but they have learnt the statistical truth: that you are more likely to sound fuddy-duddy, and fuddy-duddy is out.
So, while it's not about age but outlook and modernity, it is the case that too many older writers for young people simply refuse to "write modern." It's not about being trendy or tacky - it's about being fresh and not being fuddy-duddy, patronising and twee. The refusal to move with the times means that agents and publishers end up being inundated with old-fashioned writing from older writers.
There's only one answer: get with it, folks! Keep your ears open, and read modern books in genre. Your age should not shine through in your voice. Your age should be entirely irrelevant and invisible.
On the other hand, I'm afraid it's often rather easy to guess the author's age from the writing... You have been warned.