I think the key thing is to assume the volunteer is trained or sensible enough to moderate the advice they given depending on the age of the child (and possibly other factors). Exactly the same as they way theyd react differently to any other issue -bereavement, bullying, whatever.
E.g. if a 5 year old called them crying because their best friend elsie wanted to play kiss chase at breaktime and they didn't want to because they don't like kissing it would of course be ludicrous to suggest "do you think you're asexual?"
But if a 15 year old called saying "all my friends fancy someone but I've never felt that way about anyone, I'm wondering if there is something wrong with me or i could be asexual?" Then surely it would be okay to say "well everyone develops at different stages, you might find someone you feel attracted to when youre a bit older, or you might never feel that feeling in the same way your friends do, but that doesn't mean there's anything wrong with you or you can't live a full and happy life."
I feel quite passionate about this as I have a family member who felt this way - they got a lot of teasing "why haven't you had a boyfriend/ girlfriend at your age" "are you sure you're not gay?" Felt left out for being the only one not "pulling" at uni etc Or even the most sympathetic people only said "you'll feel differently when you find the right person."
As a result they spent a lot of time worrying about why they didn't feel what they were "supposed" to feel, and actually forced themselves to have a lot of sexual encounters they didn't enjoy in an effort to try and find this mythical "right person" or "test" if perhaps they were gay, and basically had quite a miserable ten-fifteen years until they finally came across the term asexual. Whereas if they'd known that was an option they could have just stopped worrying about it and accepted themselves as they are.
I don't get the outrage - at the very worst if you "misdiagnosed" yourself as asexual but then did find yourself experiencing sexual attraction you can just go with it - there's no rule that if you think you're asexual at age 13, that's it you are committing yourself to a life of celibacy and can never change your mind!