There is a number of factors that need to be considered when to let Turkey poults safely out on the ground. The 8 to 9 weeks rule works the best if you don't want to experiment and or take chances. This is because the poult immunity system is not considered to be fully developed until that time, also the poult need to be adjust to the normal temperature at the time they are let out.
The temperature should be reduce 5 degrees per week stating from about 97 degrees this is especially true if they were hatched in a incubator. Which is kind of hard if you don't have a hen taking care of them.
A wet poult usually equals a dead poult , so in wet climates you have to keep them dry and warm.
What potential diseases could be present. Wet ground naturally has more, some can take up to 5 years to go away, others never do.
There are a number of other reason not to risk placing the on the ground or exposing them.
It all depends what you choose to do and how many you may potentially loos.
It like waring a seat belt, you may never need it so why have it on it. But then again you might.