I'm sure for guys, this is just as important as height. How do u add one or two inches to your shoulder (collarbone) when you are young (16-18)? Is there any program for this?

good question.

anyone know the answer? i would like to know for sure

Main way i know is to stretch it.
YOu can stretch it by interlocking your fingers and pull in the opposite directions. Hold for quite a long time, you'll begin to feel the pain in your cartilage at the end of your shoulders. They should start growing.

If this doesnt work, then just increase the size of your shoulder muscle. 1inch on both sides will make you look very broad. Do bench press or any other shoulder muscle building for this.

I've been bodybuilding for a year and my shoulders have gotten wider since then.

Yes,

Ensure you are slim at the waist, and do "lat pull down" exercises and others and you will have broad shoulders. No need to worry about the bones here (if that is at all possible).

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I am a lay-person regarding medical matters. Any comments on this forum are speculative and any advice should be taken at own risk.

most swimmers have broad shoulders