Yes and, in fact, there are a plethora of adult thumb suckers who currently exist.
Get him to suck something else...
No. But don't suck on your thumb, it's weird.
They CAN suck their thumb, however by that age, they should have grown out of it.
No.
Babies mostly do
Sucking of the thumb is a bad habit that cant be stopped easily. Wrap paper tape on your thumb if it there you wontwon't want to suck it. Do this for some months. The habit should stop.
you suck your thumb
Pee in pants, Poop in pants, burp, cry, spit up, drool, suck thumb
yes
Thumb sucking at age 11 is indulged in by about 10% of those that age with the ratio of female to male being about 2:1. As such it's "normal" and, in fact, can be found in about 2% of the adult population of the United States.
Depending on the condition of the gums and jaw, usually not. Once the growth stage has finished, usually by age 14 or 15ish, an adult would have to suck their thumb, with sufficient pressure on the dental structures, more than 6-12 hours each day for years to have a remote chance of moving their teeth. If, however, the jaw bones and/or gums are diseased the chances increase proportionately and would, therefore, react to a thumb sucking force more easily but it would still take quite some time and effort.