The moment you bring your puppy home! It's ultimately not about the age of your puppy, but rather the amount of time you want them to get used to the idea of being inside a crate.
Start at whichever pace and capacity you feel you and your pup are most comfortable. All depending on their age, demeanour, and comfort level. Crate training should start as a fun but calm and engaging activity (snuffling, hiding food, hiding toys inside) to get your pup feeling curious and interested in spending time there, without any pressure.
Furthermore, make sure that your pup's crate is something they get to access to mainly when they're tired. This is to increase the likelihood that tje association with resting can be clearly formed.
Once your pup is comfortable being in and around your crate without it being closed, you can keep it closed while they're not inside, so that you open it only when they're feeling tired/in need of rest.
Saving the crate for when they're actually tired will help create that association and interest in the crate, as it will feel even more special, and just right!