If the seedlings are only a week old they should still be very small and delicate. I'd probably wait at least another week before gradually moving them outside over a several days.

I have some 3 week old lettuce starts under lights that I am going to start hardening off outside this weekend. The highs here are supposed to be around 90 for the next week so I will need to be careful not to cook them. I plan to transplant them in early September weather permitting.

Yes depends, if they... lettuce is tricky at the end of the year. Lettuce doesn't grow in a desirable way in the heat. But then some sun and wind help. Avoid hot days, and try to get them used to the outside on cooler days, with partial sun. Lettuce grows thin, leggy, and bitter in heat.

What I decided on was to move 8 of 32 containers into full shade outdoors and keep the other 24 under the lights. A little experiment to see what the difference might be. It looks like in about a week this extended heatwave will subside for a couple days. I'll be looking forward to the break and maybe starting a few transplants.

I started butterhead lettuce indoors a while back and took them outdoors into morning sun and put in shade during the hot sun. I have transplanted most of them now...very good results. I also started some in the ground recently. they are coming along great. This spot gets shade until 3 PM.