My last game in the realms took place two years ago. We started at level 0, and eventually got up to level 8. I was the DM.
The way I have always seen the Realms (and I know that this may seem direct contrast to the books) is that those level 20 characters are adventurers as much as the PCs are. And they are fighting a losing battle. If my PCs had reached higher levels, they would have found themselves facing bigger threats, and so are the NPCs in this world.
What this means is that the NPCs are in continual battle againts monsters of extremely high CRs, and continually getting killed off. For every hundred PCs and NPCs that get killed off at the lower levels, a higher level character runs out of luck.
In my campaign, Khelben died defending Waterdeep, Elminster lost his tower in a suprise attack, Halaster is dead, Mirt is dead, Elaith is gone, Storm Silverhand fell fighting in the east, Drizzt is alive only becuase he 'retired' from adventuring long ago, Manshoon is dead (evil pays the same price), so on and so forth.
Those high level characters that are still alive spend nearly all their time recruiting new adventurers and giving them quests, in the hopes that when they fall, a new generation will be there to catch the torch.
This approach has led to a very dark Realms, where evil enroaches everywhere, nations fall without warning, and there is no certainty that the sun will rise the next day.
My players liked it, and I liked it. And having all that information to start with helped me out a great deal. Perhaps some would say I didn't play the Realms in a 'proper' fashion, but we have already seen that perceptions of the realm differ widely among many.
