Hello Catherine
Polly here.  An author’s age wouldn’t really factor in to my thinking if I loved the story and felt it was pitched accurately at the child reader.  That said, it’s important to be aware that publishing houses want authors who are happy to do as much publicity as possible.  In the UK, that publicity generally involves school visits, sometimes talking to several big groups of children in a day – something many authors, regardless of age, find daunting! As long as the author is up for it, that’s fine by me.  My only other hesitation is about subsequent books.  I sign clients not for a single book but because I believe they have several in them.  So you could see why a writer of, say, 28, might appeal more than a writer of, say, 108 (though that impressive age would make a nice PR angle!).
It may surprise you to learn that I am actually more concerned about lower age groups.  I am occasionally sent queries by parents, on behalf of their children of 8, 9 or 10, who have written novels.  It’s always great to see that youngsters are bursting with stories, but I can’t ever see myself signing anybody younger than 17.   That is because the road to publication, and then being a published author, can be a tough one.  There is a lot of rejection, of criticism of work, of re-writing, of harsh reviews (in the media and online).  A certain robustness is needed to deal with these things and I would feel unhappy about subjecting a writer still in childhood to it.
Hope that helps/reassures you?
Polly