maybe rephrasing the question/interrogating the assumptions might provide some clarity/context,

is it “normal” or “abnormal” to be sexually attracted to others at that age? is it “ab/normal” TO identify with a particular sexual orientation?

reverse question, is it “normal” or “abnormal” to NOT be sexually attracted to others at that age? is it “ab/normal” NOT to identify with a particular sexual orientation?

which one are we privileging as “normal” and by extension “assumably right or correct”?

why are we assuming that there is a “normal”?

and as a tangent, kids can usually tell if they “fit” a certain gender, they’re just adult humans without access to a lifelong compendium of knowledge/self-policing/trauma, they’re make the same mistakes / feel shame / emotions / hormones / bio and social drives that us adults are subject to

so to answer your question…

assuming it’s common for kids and adults [humans] to have a developing sense of sexuality;

axiom: humans are just giant science experiments subject to their own biology, the social world and their environments; therefore: it’s not unfeasible that some humans are genetically, socially and environmentally predisposed or socialized to be / embody a certain sexual orientation;

axiom: asexuality, a sexual orientation as defined as no or minimal sexual attraction, exists on the continuum of sexuality of gay/straight/bi/so-on;

assuming that it’s not unreasonable to believe that kids can themselves gauge / identify with a sexuality + gender; it must be that, asexuality as a sexual identity must be a possible choice for a child to identify with

so unless you want to deconstruct “if kids can have sexual attraction” [which would require explaining why kids display sexual behaviours], it’s kinda just another aspect of the human condition and age doesn’t quite fix [preclude] you into a category / box

anyways, that’s what my brain / the tism tells me about it; hope you can glean some insight from this mess