I didn't actually forbid either of my sons from swearing.
I tended to swear like a sailor when I was younger. When my first child was 2 years old and used some four letter word he had heard from me, I took a few seconds to think about the fact that a) he was wired to mimic me, that was how he was learning to talk, so it would be cruel and counterproductive to punish him for doing so and b) my language wasn't going to be miraculously cleaned up overnight. So I turned to him and said "That's a bad word and I shouldn't use it either. I will let you know what the bad words are and when you forget, I will remind you not to use them and when I forget you can remind me not to use them."
So my two sons spent their childhood trying to clean up my language. My oldest never swore and by the age of 11 had an extremely obnoxious superiority complex about it, looking down on anyone who used such language as being too stupid to come up with a better means to express themselves. (This led to me giving him a lesson in reasons why such strong language is sometimes the only way to adequately express something.) My youngest, who is more strongly emotional (just like me) was still inclined to use faux-swear words (like "darn!") when he was frustrated or what not but really didn't take up swearing until his late teens.
This off-the-cuff policy that I thought up in mere seconds was so wildly successful in discouraging my kids from swearing that I spent a lot of time wondering why. I ultimately concluded that forbidding children from using swear words and reserving it for adult use makes cussing attractive as a power play. It's a way of grabbing power and announcing you are all grown up or as good as the adults or some such. But telling mom to clean up her mouth was a much more genuine exercise of power over me, way cooler than cussing could possibly be. So I effectively removed almost all temptation, except for the intriguing detail that my youngest still found something emotionally satisfying in strong language even without the allure of defiantly displaying some forbidden social power.
When they were school age, I would explain what a word meant and that using it at school (and some other social situations) was likely to cause fall-out they didn't want, but still did not forbid them from using "bad" language (including words that aren't swear words but are considered simply "rude"). They didn't want to be hassled by a teacher/other adults, so they watched their mouths -- not because I required it, but because it was in their own best interest to not attract trouble.
I tended to swear like a sailor when I was younger. When my first child was 2 years old and used some four letter word he had heard from me, I took a few seconds to think about the fact that a) he was wired to mimic me, that was how he was learning to talk, so it would be cruel and counterproductive to punish him for doing so and b) my language wasn't going to be miraculously cleaned up overnight. So I turned to him and said "That's a bad word and I shouldn't use it either. I will let you know what the bad words are and when you forget, I will remind you not to use them and when I forget you can remind me not to use them."
So my two sons spent their childhood trying to clean up my language. My oldest never swore and by the age of 11 had an extremely obnoxious superiority complex about it, looking down on anyone who used such language as being too stupid to come up with a better means to express themselves. (This led to me giving him a lesson in reasons why such strong language is sometimes the only way to adequately express something.) My youngest, who is more strongly emotional (just like me) was still inclined to use faux-swear words (like "darn!") when he was frustrated or what not but really didn't take up swearing until his late teens.
This off-the-cuff policy that I thought up in mere seconds was so wildly successful in discouraging my kids from swearing that I spent a lot of time wondering why. I ultimately concluded that forbidding children from using swear words and reserving it for adult use makes cussing attractive as a power play. It's a way of grabbing power and announcing you are all grown up or as good as the adults or some such. But telling mom to clean up her mouth was a much more genuine exercise of power over me, way cooler than cussing could possibly be. So I effectively removed almost all temptation, except for the intriguing detail that my youngest still found something emotionally satisfying in strong language even without the allure of defiantly displaying some forbidden social power.
When they were school age, I would explain what a word meant and that using it at school (and some other social situations) was likely to cause fall-out they didn't want, but still did not forbid them from using "bad" language (including words that aren't swear words but are considered simply "rude"). They didn't want to be hassled by a teacher/other adults, so they watched their mouths -- not because I required it, but because it was in their own best interest to not attract trouble.