Chicks can go outside for short periods starting around two weeks. This helps to acclimatize them to cooler temps as well as provide more opportunity to flex their little growing bodies.
Beginning at the end of the first week, you should be weaning chicks off heat by reducing the temperature of the heated zone. By age three weeks, chicks should no longer require heat during the day as long as the ambient temperature is around 70F.
If you keep reducing the temperature of the heated zone, chicks should be well feathered around five weeks and they can go live in the coop with no heat if the night temps aren't below 50F, and if they're colder than that, minimal heat or a snuggle box should be all they need to get them the rest of the way weaned off heat.