I have a case that involves a victim that does not remember exactly when the crime happened, but can give a ballpark figure. The juvenile who did it turned ten years old during this window of time. Although the juvenile is very young, I do not think that this in an appropriate case for a deferred. Is the issue of age simply a question of fact for the jury to decide, or is it jurisdictional?

I think minimum age is jurisdictional. See section 51.042.
Dawson has a pretty good discussion of this issue in the TJPC juvenile book but I don't think any of the cases he sites are directly on point to your issue.
I do know in Ft. Bend Co. we had a least one case tossed by the Judge because our proof on that issue was insufficient (I still love ya Bob !).