It's generally advisable to get hermit crabs when they are small but old enough to feed and care for themselves. This is usually at least the size of a small marble. These smaller crabs are easier to handle and may adapt better to life in captivity. They will also have a longer life expectancy since larger hermit crabs are often older and may have already lived part of their lives in the wild.
Keep in mind that caring for hermit crabs requires dedication and proper knowledge about their needs, including temperature, humidity, diet, and substrate. Researching and setting up a suitable habitat is essential for their health and well-being.