Hi. Other than the fear of getting caught, you may also like to consider that there's actually a reason why there is an age restriction for marathons in major races (including for Boston, London etc). The theory is that as teenagers' bodies are still developing, they run the risk of injuries due to the intense training required and overuse (a lot of pounding involved in running). And males develop later than females.
You can google for more info (including for Osgood-Schlatter disease) as there are many articles on teenage running available online.
As for myself, I did an MRI once and the doctor spotted an abnormally in my knee. It doesn't cause me any pain and it has never bothered me. But the doctor asked me a very strange question - did you run a lot during your teenage years? i did, I ran a lot while in school. That's when i found out more about this.
Im 17 this year and I ran 3 HM before with army half being my 4th so I suppose I would like to try this upcoming sundown marathon since it's cheap.
Honestly the fear of getting caught with someone else's bib is preventing me from running one
It might be good to check with the organiser's as there must be some reasons for them to keep to 18 years old as the minimum age.. might want to try out the shorter distances.. did u run shorter distances such as 10km and HM before?