Where is your source? I have NEVER heard this. Roosters are killed frequently at 16-20 weeks though.
It's a well known fact, as Mary states. The reality is that most shipments are pullets. There are no magic happy ever after endings for the millions of cockerels that are hatched every year.
Lili, if you hatch chicks, the sad fact is that at least half of your chicks will be cockerels. Occasionally, you will get lucky and have more pullets. But PLAN on having 60% of your hatch being cockerels. Realize that the feed conversion rate for those cockerels to raise them up for processing is poor when compared to birds that are specifically bred for meat. Therefore, there is not a market for them. You will be lucky to be able to give them away. When you give them away, you loose all control over how they are treated. They may be used for dog food, cock fighting, to feed someone's snake, to feed captive raptors... or simply treated poorly. That is why many of us choose to raise our own cockerels, and process them for our own freezers.
If you don't have an exit or long term plan for all cockerels that you hatch... And if you aren't willing to cull any deformed or failure to thrive chicks that you hatch then, you should not be hatching chicks.