I guess that you have to go with the rules that the competition sets although I must admit, as the parent of skipped kid in a B&M school, I do think that they should use consistent guidelines and they often do not. For instance, back when my dd14 was in middle school (she just finished 10th grade), she did place in a few competitions such as writing and talent search, but not as highly as she might have had she not been a 11 or 12 y/o versus others who were in her grade and, say, 13 or 14.
The thing that I guess is hard to account for is subject acceleration as well. There were, for example, kids at the talent search awards who were 14.5 y/o 8th graders who were either online schooling utilizing 9th or 10th curriculum across the board or in B&M schools where they were subject accelerating multiple yrs while still enrolled as 8th graders. Keep in mind that I do have a subject accelerated kid, too, so it isn't that I am opposed to subject acceleration. It is just a question of whether you can fairly compare achievement of a kid who has had more opportunity to learn with a child who has not had that opportunity.
I guess that I have no answers here! We're thinking about this too as of late but on higher stakes things like national merit for which dd will be testing in the fall next yr. She very well may make it (her current national test scores like PLAN bode well for her being in the 99th percentile+ compared to her grade mates), but I know that, if she does not, it will likely have a good amount to do with her being much younger than other 11th graders locally and nationally. None the less, it was still the right move to put her where she is.