There are cases of people with no sense of pain. Here’s an article about it:
http://findarticles.com/p/articles/mi_m1200/is_25_170/ai_n26713199/
...and a link to the primary it references, if you have a subscription to Nature:
http://www.nature.com/nature/journal/v444/n7121/full/nature05413.html
A relevant quote from the review:
“The team’s first research subject, a 10-year-old boy, was well known in his community for street performances in which he placed knives through his arms and walked on hot coals. Despite tissue damage, he apparently felt no discomfort.”
Other mutants for the same gene were similar—they generally enjoyed showing off their lack of pain by deliberately injuring themselves. One killed himself by jumping off a roof, I believe just after the above article was published (sorry, I’m not really sure where I read that). Living without pain is generally an unhealthy idea. Without visceral negative feedback, humans don’t place value on bodily integrity and tend to self-terminate, which is probably why those mutants are incredibly rare.
Perhaps once aversion reactions have been formed, knocking out pain receptors would be a good thing. I’d certainly be curious to see if people still avoid injuring themselves if they lose the ability to feel pain AFTER they’ve experienced it all their lives.
Also, vitamins deficiency might set in at different times for adults and children. Children grow a lot, so their nutritional needs are probably different from adults.
No source, just speculation.