If I understand correctly reading the manual, the $allowHyperlinks=10 should not convert URLs posted by anyone with less than 10 posts? But it does.

Not by anyone. Admin and moderators are not in this list.

If you are sure it works, try to make some posts on miniBB support forums here :)
We have the same rule applied in both places. You will see it doesn't. It may be some misconfiguration on your end.

In our demo, it's allowed to post hyperlinks by anyone. The website has NOINDEX,NOFOLLOW meta tag and it should not be indexed in Google anyway, so it won't work for spammers.

I too am having the same problem - I have set $allowhyperlinks to a value of 5 and then to 10, but links still get posted and are active.

Also, if I look at the Page Source for my forum (and this form too) I see <meta name="ROBOTS" content="INDEX,FOLLOW" />.

Should this not be NOINDEX.NOFOLLOW ?

Ok - not so true about hyperlinks - I set a separate user for myself (not admin or moderator) and I was allowed to post an active link, but when I got another forum user to post a link it was not active...

Weird.

Ah - I wonder...
Before I made the changes to the $allowhyperlinks to value of 10 I had already posted a profile with a hyperlink that was active, as it was allowed at that time.

Does this get remembered, and then means changing the $allowhyperlinks value only affects new registered users from that point on?

$allowhyperlinks affects only amount of posts. As soon as you have done [$allowhyperlinks] posts, the hyperlinks you post will be highlighted.

It's not up to user's profile. The function doesn't count what you posted in profile (and in general, I see nothing common in profile and messages).

As I wrote above, this rule is only for regular members. Admin and moderators are free to post hyperlinks anytime.

All message threads should have INDEX,FOLLOW meta. Study SEO guides for more.