I see you have varying responses, but I think that over 3 would start to be damaging to teeth (at least that is what our dentist said about thumbsucking).
I have to admit that I sucked my fingers (only at home, mostly night) until I was around that age (I did have a difficult childhood). Anyhow, around that age is when kids do sleepovers, etc. While I am not the most socially adept person, I did break myself of the habit because I wanted to sleepover at friends' houses, etc.
So at 8, there would be health (dental) as well as social ramifications. I wouldn't get into as just a friend, but if I were the dentist, doctor, teacher, etc., I would advise that it is not healthy or normal.