I would say start as soon as you'd like. Yes, 49 is going to be a good bit better than at 43, but you can still make leveling from 1-20 ridiculously faster for your friends than if they're grinding out the levels on their own.
As far as locations, imo the key is finding unpopulated areas. Unrest is a great zone to PL in, but it's too crowded. Befallen even lately has had people, so I've found alternatives. 1-10 I've been doing sro, cause there's a big desert full of mobs and no competition. 10-20 I've been doing runnyeye because there's never anyone in the zone. 20-30 I've yet to identify a place I'm entirely happy with.
One thing I didn't see Daldaen mention, and maybe it's intuitive, but when tanking low level mobs, repeatedly sit after the player has hit each mob, as it will let the mobs automatically hit you. They'll die a whole lot faster.
-Laithus
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