Sorry for the late response. But I've been at a friends all weekend.
Are you going by just how serebii has it @ 7 only in the pokedex? Cause if so, it's flawed (not being mean, but nothing's perfect) which is why I use multiple sources... Seeing as bulbapedia, marriland, and 3 seperate videos on youtube of people encountering lv 6 larvitar (one being shiny he chianed), the TTar I have mentioned below, and I asked people with diamond to go find larvitar @ lv 6 in their game... pretty sure it can be found at 6.
I checked about larvitar @ 6 from 207 a good while ago when someone traded me a shiny tyrannitar
Kinda moot since he said his was met @ 5 anyways. Just didn't want people to get the idea that the ones @6 are as well.
Hmm. That's strange since my friend chained Larvitar a while back and all 10 shinies were Lv7. (No-one payed attention to the leval of the non-shiny ones) Do you know the persentages of each? Maybe Lv6 ones are just have a realy low chance.