Teaching 3-5 week old puppies to mand is very easy. Their pudgy bodies end up in a sitting position frequently, which provides ready opportunities to click and reinforce the behavior.
Working with pups older than this is more challenging because they have lots of learned behavior and muscle memory in moving around as puppies do rather than thinking about how to get a person’s attention.
The other challenge can be getting and holding their attention because they are much more independent than younger puppies are.