Answer: It's not too late! Leaf lettuce (as opposed to heading types) matures to a harvestable size fairly quickly--some types in as little as 45 days. I'd go ahead and do a small planting now, then do another planting every two weeks for the next 6 weeks. Lettuce thrives in cool weather and can even take a light frost--so you'll should be able to harvest tender, young lettuce well into late fall.
Q&A Library Searching Tips