Girls, whose children suck their finger? My friend's son is 8 months o…

Girls, whose children suck their finger? My friend's son is 8 months old and he constantly sucks his finger, even when he sleeps, is that normal? Well, my son also in the mouth hands put, but at 8 months already big, count crawling and then sucking his finger. In general, from what is it? I wouldn't want my son to suck his finger at such an age, she asked her, she doesn't know