My ds is 8 weeks old. At 6 weeks he sort of smiled and now will smile, but it takes a lot of work! It isn't much in reposnse to us, but he will do it.
He is very alert and loves to watch what is going on. However I saw a friend when her baby was 9 weeks and he was smiling, giggling in reposnse ot songs and peek-a-boo. This is repeated amongst friends I know.
I feel really bad that I am doing something worng to make my ds so slow in developing.