Yea I think if you don't give the baby a pacifier they will start sucking on hands/fingers etc. my friends baby is like that and she is 3 months.
mine never did, she's about to be 11 months old. She puts her fingers in her mouth and chews on them, but I've never seen her suck her thumb.
a week old. but I stopped him. he's 2.5 and he'll suck his thumb every once in awhile. but I think that's because he misses his Binky
at 3 months 😊