Its completely fine, you really didn't screw up big time.
I give them Purina Start & Grow until they are 16-20 weeks depending on how many other chickens I have of different ages and then I switch them to the Purina Layer crumbles.
I have all different aged chickens that get integrated from the grower pen to the small coop and then into the main coops so I can only do the best I can to get them on the layer crumbles at the right age but sometimes I have an overlap where its not possible and they start a little earlier.
You didn't switch them over ridiculously early so don't sweat it.
As you said they free range so they are getting all kinds of other protein.
Mine free range too and I also feed all of mine black oil sunflower seeds, I throw a some on the ground for them once a day and sometimes I mix it into their feed as well.
I also feed them yogurt once or twice a week which is good for their gut.
Both of those give them extra protein and as I said the yogurt is good for the gut (they especially need it with all this nasty wet weather we have and the ground being so mucky) and the black oil sunflower seds make their feathers nice and shiny as well as being a healthy treat for them.
Remember to add oyster shells to their food or throw it out to them for free choice so they have nice strong shells otherwise you will have thin shells that break easily.
I just mix mine in with the crumbles.
I give them all of our left over pumpkin guts after carving them out near halloween.
I scoop everything out and give it to them, they go wild overit.
Pumpkin seeds are a natural dewormer and the pumpkins have alot of vitamins.
When the fall season is over I take all the pumpkins and just split them open and leave them to eat out.
As far as their age, the majority of mine started laying between 20-24 weeks but I have had some that were earlier or later.
I have some pullets that just started laying last week at 20 weeks exactly but I have others, usually my easter eggers that have started as late as 28 weeks.
You can tell when they are starting to get ready to lay, their combs and wattles get bright red and they crouch down when you go to pat them.
Its a subservient pose they do, its the same one they do when the rooster wants to mate with them.
Mine usually started laying within 2 weeks of those signs.
Hens will crow sometimes, especially if there are no other roosters in the flock.
The signs of a rooster aside from having the larger wattles and combs is the saddle feathers on their backs.
While the hens have thick, straight, fluffy feathers at the ends of their backs, right before their taies the boys have thinner, longer feathers that drape down towards the ground.
They also have the long curving tail feathers and the longer legs and bigger feet.