I just started FONV and bought all the DLC, so I got three additional quests in my log. But can I start those quests immediately or should I wait until I'm much higher level?

OWB is allot easier at levels 30-39. You'll face mostly hunting shotguns and hunting rifles that still sell for good caps. At level 40+ you get fisted by .45-70 Gov't weapons wielded by Lobotomites that shoot for the head.

Ok thanks. I'm like level 8 now. Guess I gotta wait till the end of the game then.

Don't wait till the end!
But make sure you have high skills for Dead Money. I've gone in around 18 came out fine.
OWB you may want to go in high leveled.
Honest Hearts- depends on whatcha want to do, although, you may want a high speech skill!
:tops:

I started Honest Hearts at around level 10, and leveled up sniping Yao Guai and cazadors. Dead Money should not be played before the 15th level as you usually need some high skills to enjoy it. As for Old World Blues, level 25 is what I recommend the earliest entry level should be unless you are super sneaky with some equipment that has a stealth field on it. :ph34r:

If you want to farm high end weaponary and ammo, then definitely wait till you're above level 20 for HH and OWB. White Legs and Lobotomites drop some really expensive hardware e.g. Brush Guns, AMRs and Hunting Revolvers. Dead Money doesn't follow this rule since pretty much all the weapons you find there are DLC exclusives, but all the environmental hazard will be hell for a low level character.