Maturity comes with size and not age. They all grow at different rates due to variations in diet and where they live, so we can't really say when they become adults.
I wouldn't want to leave them outside all day until they were around 400-500 grams. And even then it would be weather dependent.
It's not just the minimum temperature, but as Tom said, the stability of the temperature and humidity. IN the sun, or under a light, the humidity in a tank or enclosure can drop significantly, and these tortoises need high humidity to stay healthy. IN the wild, they would be pushed deep into high grass or bushes, still wet with morning dew, or the afternoon rains. They thrive in those humid conditions. In an artificial enclosure, it will be much drier than that unless you specifically do things to bring the humidity up. They also will suffer greatly from stress if not given hides in which they can feel secure. I put pots of overhanging plants with mine, and they spend most of their time pushed under those overhanging plants as they would in the wild. Secure, and more humid. The pots are heavy enough to not be tipped over, and high enough so they will eat what they can reach, but can't get to the whole plant. In my enclosed chambers, I hang a boston fern from the top of the enclosure. Boston ferns do great in indoor situations and artificial light - and is good food!. They can walk under the hanging pot, but the fern drapes over a large area. They will nibble it and trim it, but it creates a nice overhang.I live in india, so the temperature never drops below 80, and I am getting and enclosure made soon.
