They can go outside for a short while any time you want, even when it's chilly out in early spring, as long as you take them back under the lights as soon as they look chilly and don't leave them out long. Remember, if they were with Mama Hen, she'd have them out in the snow at a week old, she'd just call them back under her wings when they got cold.
Your babies can easily go outside at 88 degrees, but will have to go back under the lights when it is chilly at night. I wouldn't let them out full time at only a week and a half old--just when it's nice and hot out, or for short periods when it's cooler.
Or, you can do what we do this time of year, and rig up a heat lamp in their shelter outside. Then they come and go under the lamp as they please. We have ours fenced so they can't wander too far away from the light.