Level I adventurers can not take jobs meant for level V.
Higher level adventurers are also not allowed to take a level I adventurer along when undertaking level V quests.
Level V quests pay quite a large sum as compensation, but those are mostly situations where their lives are at stake. The prohibition exists so that they wouldn't become a burden and because there are cases where, at worst, the precious level V talents meet their deaths.
On the contrary, level V people can take on level I jobs without any problem.
No penalties are imposed.