I would say between 15 and 20. Do you happen to have a side shot of the teeth? The angle of the teeth could tell us more - a very young horse will have very up and down teeth but as the horse ages, their teeth will begin to "buck" out.
The Galvayne's groove is only accurate to a point. For instance, my mare's groove is totally gone - indicating she's over 30 - but in reality she's actually only almost 28 (she could even be 25, if the age on her sale paper is wrong and what her previous owner verbally told me was right - I choose to go with the sale papers though).
Here's a link with showing several horses that, according to their grooves, are much younger than they are actually: Galvayne's Groove In Horses: Photos
But yeah, in any case, I would be seriously surprised if this horse was under 15.