Rule 10: first sentence indicates that the distance limits (ie the Rule) override what a race organiser might suggest for his race for anyone under 18 on 1.1.2012.

Once you become 18 on the day you literally cease to be a junior and Rule 10 does not apply.

However if you were competing in the "Under 18 and Over 16" Junior Champs because the key date is 1.1.2012 you would still be eligible to be counted as "Under 18" Category for the rest of the 2012 year.