I used it with both mine - DS from 4 weeks, mind you he was born 3.5 weeks early - it worked a treat for him and would really help bring the wind up, I did try Infants Friend and some other natural remedy type wind drops first, I think they were Braurs? But they made no difference...
With DD I used it prior to 4 weeks, however it didn't really seem to make much difference for her but she was a spewy little thing