My 3.5 year old has the habit of thumb sucking since he was an infant. Is it OK if he keeps the habit or is it bad for him in any way?
Hmm. My son asked me the same thing this week. I wrote him this.
"Most children should grow out of thumb sucking and pacifier use between the ages of 3 to 4. As long as the habit is discontinued well before their permanent teeth come in, your child should be fine." source http://www.dentistry.com/daily-dental-care/pediatric-dentistry/the-effects-of-thumb-sucking-and-pacifier-use-on-teeth
"The middle teeth are usually the first to go (at 6 to 7 years), followed by the ones on either side (at 7 to 8 years)." source http://www.babycenter.com/0_your-childs-teething-and-tooth-loss-timeline_10356447.bc
I would suggest they go to the dentist to determine if harm is being done, otherwise, don't worry about it. It isn't always a problem.
My daughter sucked her thumb straight through to grade 2. Nothing we tried to stop her worked. Our dentist said that the way she sucked wasn't affecting her teeth. She never did need braces and has beautiful teeth. The sucking was a relaxation activity, and drove her teachers and the parents of other students crazy with worry that others would copy her, but it wasn't a problem for her. She also had lots of friends and got along very well at school. I'm not just saying this to counteract the previous poster's response. It is our real experience, and was contrary to any popularly help beliefs. I'd suggest to take him to the dentist to determine if a problem is developing. If it is, he needs to stop. If it isn't, he will stop when he has learned other means of relaxation.
It can cause teeth to move in the wrong directions as they grow in, so in that respect I would say it needs to stop by the time permanent teeth start coming in or you may be looking at some pretty good dental expenses (braces). Aside from that, the biggest concerns would be getting teased for it once he starts pre-K or Kindergarten and chapping on his thumb from constantly being in a moist environment.
Hope that helps!