Um…..sorry, not necessarily.
Simply Google “adult thumbsucking” and you will get 42 000 results.
There are websites dedicated to this habit.
Famous suckers include Rhianna and Amy Winehous
If you ask around, guaranteed, you will find someone with a grown up son, a cousin, a hairdresser or a husband (I am NOT kidding) who STILL sucks their digit in adulthood.
Adults simply become better at hiding their habits, and it is often used as an emotional crutch or simply for relaxation.