I walk two in the morning, four in the afternoon, one in the evening. It is almost unknown to the old man, but to the little children, it is simple.

The answer to this riddle is the number 7. The riddle is describing the number of legs a person has at different times of the day - two legs in the morning, four legs in the afternoon (if they are using a walking stick or cane), and one leg in the evening (if they are sitting down or lying in bed).

The answer is "the letter M".