My father told me he read this story when he was young (I think). He was born in 1935 and is passed away now. He didn't remember the title or author but it impressed him.
I expect this is probably Deathworld (1960) by Harry Harrison. The protagonist is Jason dinAlt, a mildly psychic gambler/con man who is recruited by Kerk, a citizen of the titular planet (actually called Pyrrus) to win a bunch of money to buy guns, and who is forced to flee to Pyrrus to escape the casino bosses who come after him.
Pyrrus is indeed extremely hostile, with all the flora and fauna constantly evolving to better attack the human colonists. All the colonists always wear guns in power holsters that can put their gun in their hand in a fraction of a second. Jason is a bit of a quick-draw expert, but is no match for them. Children are taught to use weapons from a very young age, and in fact Jason is put in with some very young children for remedial weapons training.
The full text is available from Project Gutenberg.