I don't know!
All of my adult gardening life has been in Zone 9b, and I only grew a few annuals from seeds before that (in the Midwest).
Obviously the seedlings will have to have some kind of decent root system, and for that, they have to have some decent amount of foliage. I'd think if you tried planting out something that was very small, with only 5 leaves or so, and not much of a root system (not enough to fill a 4" pot), that maybe it wouldn't survive.
At this point, I'd say go ahead and start them - what do you have to lose? If you wait until winter to start them, they may have lost viability, as I understand it.
The earliest frost you would get in Zone 6a would be about October 15th. One month before that would be September 15th. If you start seeds now, and if they germinate within the next 2 weeks, that's 4 1/2 months of growth before they need to be in the ground. They might be big enough, IF they aren't all crowded growing together in one small pot. (I'd get them into the ground a lot sooner than September 15th - as soon as possible after they are up, and then watch them like a hawk.)
If, on the other hand, you have to stratify the seeds first... I dunno.
Maybe someone from a similar zone to yours, who has had some late seed starting experience, can chime in here...
