January 1st and Thoroughbreds

On January 1st all Thoroughbreds and Standardbreds (and probably others) celebrate their communal birthday and turn another year old.
In this age of computer records the actual birthday is usually available as well.  At the very least most horses have a verbal record of their history that follows them through life.  It might not be terribly accurate, but it tends to be fairly consistent.
Nina has none of these.  Oh, she has a tattoo on her upper lip, a combination of letters and numbers that indicate that she is a Thoroughbred and at least went to the track even if she never raced.  It also tells her birth year.
The problem is that the tattoo is illegible.  Several vets experienced with track tattoos have given it a good shot.  Many others have tried.  As one vet put it.... she was apparently never considered a potential star, it's a cheap tattoo, not done to be long lasting.
Horses can also be aged by their teeth, which changed drastically every year until the age of ten, when they don't change much except for wear. 
Her previous owner told me that they did decipher the tattoo several years ago, when she first bought Nina and sent the info off to the Jockey Club to get her history.  The information came back on a "dark bay or dark brown" filly.  They discarded this info, assumed they got the number wrong and forgot about it.
They didn't know that most black Thoroughbreds are registered as "dark bay or dark brown."  The JC is reluctant to register a horse as black because black is genetically a different color and getting it wrong could mean questions of parentage in future generations where a horse turns out a color it shouldn't be.
Nowadays you can do DNA testing and find out the genetic roots of your horse's color, but "dark bay or dark brown" is the well used and safe option on JC papers.
Her previous owner also handed me assorted Coggins tests on Nina (or not) that make her anywhere from 12 to 19 at the time I bought her.
So why is this an issue?   It's because I am making plans to compete with her later in the year and competing means filling out entry forms and that means entering an age.  Horses with an unknown date of birth and over the age of 10 are considered 'Aged' (another horse specific term).  But, again, in the era of computers...'aged' doesn't fit in that slot for a one or two digit number. 
I think Nina and I are a pair of dinosaurs, caught between traditional descriptions and modern (unforgiving) computer records.
When it comes to actually filling out forms, I will just ask a couple of event organizers what their solution is.  I have a feeling that I will need to pick a reasonable age and stick with it.
I wonder exactly how vain Nina is.....this might be a good bargaining chip.  A flattering age for good behavior?