ok-- so i saw your question and said hmmmm. i have a daughter who, at 6 months decided to take out the pacifier, which she was never fond, of and suck her thumb. At first, i giggled. then she began to do it at night to comfort herself. we went on vacation when she was 18 months and she got restless on the plane, so i told her to suck her thumb. she got super sick, (fever, vomiting) while on vacation and began to suck all the time. she continued to suck all the time at home, while playing with toys, etc. she was young enough that i didn't freak out about it, but i saw it as an increasing problem
A little about me: I am a dental hygienist and a 28-yr-old mother of three small children with a husband also in the healthcare field.
so-- i know your pediatrician will tell you not to worry. it is normal, and he will most likely grow out of it. The truth is, he is right. The problem is, I have seen the worst of what habitual thumb sucking can do to your facial structure and the habit it can become. not to scare you with all the things I have seen, but it is not uncommon for 25 yr olds to suck their thumb, because it turns into a habit such as tapping your feet while on the computer--- you don't even know you are doing it. some people suck in a manner that will affect how the roof of their mouth is formed and how teeth will sit. others suck in a way that will not affect these things at all. then you just enter the social stigma of your 12 year old sucking his thumb!!!
enough of that- this is what my husband and I decided. rather than enter all these " what if's " we gave our 19 month old daughter a "special sock" and night time and nap time!! we put a onesie on her and her brother's tube sock over the top of it and very gently used athletic tape around the top and then put her shirt over the top of that. (no tape was on her skin, but she couldn't get her top shirt up high enough to take the tape off) her special sock then became a need for night time-- even when the thumb sucking had passed. it only took about three weeks to see that she didn't need to suck her thumb during the day. we kept the sock going and nap and night time for a couple of months, just to be sure, but she wanted it. i am glad we did it, just because it is now no longer and issue at all and she will not have to deal with that. kids have enough to deal with as they get older. I really hope this helps. It did for us!!