Er... DM stands for "Dungeon Master", the player responsible for outlining the plot and setting in tradtional tabletop RPGs, Mizzi. [/nerdiness]
Ahem... I'd say that as far as starting out goes, trainer plots do sometimes never get off the ground because of the whole "starting with weaklings" thing. I mean, (not to advertise, but) in Neo Genesis, starting with wet-behind-the-ears newborns ain't such a bad thing because our characters are A) enveloped in a completely crazy plot, and B) Have other accomplishments outside of training Pokemon to balance their lack of experience as a trainer. In a lot of the "n00bier" trainer journeys, you aren't given that choice. Your character is ten years old, knows nothing, and is randomly given a Pokemon who can't win cool battles and won't obey him for heaven knows how long. It starts to get boring, and roleplayers lose interest. So I'd say that it is usually a safer bet to let the roleplayers have some leeway in age and accomplishments rather than have them start off as just useless ten-year-olds who barely know what a Pokedex is. XD