The weather here is just beginning to turn - 2 more days of 90-degree highs and then a drop into the 70s/80s with lows in the high 50s or low 60s. I'll be planting lots of leaf lettuce Thursday morning if this forecast holds true and sow more next week. I sink a few supports (garden stakes or even sturdy sticks) at the corners and in the center of the lettuce beds and drape old sheets & blankets over them at night when it frosts. This has been surprisingly successful in past years and I'm often able to pick quite a bit of beautiful fresh lettuce for Christmas dinner. Snow won't kill it off but I've found that no amount of "covers" will save it with sustained temperatures under 20°F, although it can definitely survive a quick early-morning zap.

Green Ice is my favorite to string along like this, but good old Black Seeded Simpson takes to it, as well. Salad Bowl is iffy.

In mid-October I'll be sowing seed into containers that I can haul inside. (I love green leaf lettuce!)

Um, to actually answer your question, in my opinion no, it isn't too late for you to plant leaf lettuce, but it may (or may not!) take some babying. As for when early fall commences (weather-wise), I "feel" that our upcoming cool-down is the real deal and does signal the beginning of Fall, but that's here. We've had leaves falling for nearly 3 weeks now, the corn is drying at warp speed, everything in the garden seems to be rushing to ripen, and the dew is becoming heavier each morning. This may not be the case in your region. Sorry to be so vague!