my youngest sucks his thumb, he started doing it at about 7 or 8 wks old, when i had my 20wk scan in pregnancy he was sucking his thumb, i have a very cute scan picture of it
he is now 18mths old and still sucks his thumb, and has a cuddly blanket, he uses them when he wants to go to sleep, if he is tired, teething or a bit fretful etc.
he is a happy normal toddler, like my elder three were, with the added bonus that as he sucks his thumb he can soothe himself to sleep
i sucked my thumb as a child and grew out of it eventually.
i would just get rid of the dummies, they can ALWAYS find their thumb so no getting up in the night to play hunt the dummy
i think most kids do just grow out of it, or if you want to discourage it you can put that yucky tasting stuff on their thumb (like to stop nail biting) but imo thats cruel, sucking is a natural comfort to babies and they will stop when they are ready