How should I write Age.h? Is it almost the same with Age.msg?
update: After all, I found it works even with/without empty the header file.
You don’t need to write Age.h because it’s automatically generated when you compile the Age.msg message. Also, the header file is not empty, only that it’s not in the same location as Age.msg.
Don’t worry, if you just follow the examples and instructions given in the notebook, the right headers will be automagically available 
.