First why at 13 weeks!

Help me out!
We have 3 Sussex and 2 copper marrons.
Im scared one of the Sussex is a rooster :( but thats not what this is about.
Today, they are 13 weeks old and we got the first egg! They werent supposed to lay untill
20-25 weeks.
We feed them chickfeed not layer feed and they scavenge. They eat Shells, snaaks etc. So maybe they do get a lot of extra calcium we also give buggs and general greens (grass, appels, veggies)

Is this really bad? We were told not to give layer feed to soon because their tract (?) would develop too fast and it wouldnt be good but now its the first egg??

The one Sussex I suspect to be a rooster also has a lager comb and lobes… and obv is my fave:(