I first started putting mine (Southern Painted) out at age 3, but I guess any age would be OK once they are past the hatchling stage as long as the water is warm enough.

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2.3.0 Southern Painted Turtles
1.0.0 Florida Red Belly
2.3.0 Common Musks
1.0.0 Classic Cornsnake
0.0.2 GALS + infinite babies
0.1.0 Tibetan Spaniel
1.2.0 Brahma Chickens