20-24 weeks is AVERAGE for beginning to lay, thus the recommendation on the bag. Frankly, that is quite accurate. Many birds fed organic reach point of lay a week or two later anyhow.
However, some, faster maturing breeds start laying at 17 or 18 weeks, although this too varies by individuals. Not everyone starts the same day.
So, depending on what you want to do, follow the recommendation on the feed back (which I would) or start them on layer a bit early. I assume you're getting ready to buy another bag of feed? Just remember that Layer is nothing more than regular feed, with calcium ground right in, something you can easily accomplish with oyster shells offered on the side.