It doesn't matter how low level they are, they can still drop the shards.
People keep saying this, but I'm not seeing this as the case.
I always get shards for instance, grinding SP on low ranked classes in the lowbie parts of Thanalan. However, when I kill these sort of mobs on my 24 Marauder, I never get shards. I've been killing them randomly when I felt like it to make sure, and even just now I went and killed about thirty Bumble Beetles / Star Marmots. Not one dropped a shard or a crystal.
What I'm saying here is not proof, but I'd invite people to test this further. I tentatively believe there is a point where you're too high above the mob to get shards/crystals (or at the very least, the drop rate becomes insanely low), and that people are only stating otherwise because they aren't high enough yet to experiance this.
Edit - Forgot to include that my physical level is 42.
I should also add that this makes sense, as it did in XI, as you don't want a bunch of highly ranked players 1-shotting all the easy mobs that should be going to the players on low ranked classes. Edited, Nov 7th 2010 6:29pm by Coyohma