I had new grass 16 days old, growing in a cup indoors. It was Scotts perennial rye and it was about 3 inches tall.
I sprayed it with Quicksilver (contains carfentrazone). The spray solution contained about the equilivalent of about 2.4 pints per hundred gallons of a non ionic surfactant--which the label suggests may reduce selectivity.
At 24 hours some of the sprouts had fallen at an angle. At 4 days more than 50 percent of the new grass blades had tipped at an angle and were no longer vertical. It appears that a sensitive rapidly growing area near the soil line had been affected, and perhaps the rapidly growing cells had collapsed.
The label says that new seedlings may be treated 7 days following emergence, but mentions temporary yellowing on hybrid Bermuda grass.
This indicates the label should be believed in this case. Newly seeded grass can be treated, but 7 days after treatment is perhaps a bit optimistic.