Technically, your not the one renting the marker. When you go to a field (in the US anyway, I'm not sure about overseas), if your a minor you need a parent/guardian signature anyway (you can get away at most fields with someone who can prove their an adult). The field technically rents the marker to them, who in turn allows you to use it.
And I'm not sure about the 18 years to buy equipment either. The only place I've ever been carded for equipment is wal mart, but its store policy to not sell specific stuff to minors (ie R rated movies, 17+ videogames, paintballs, etc).