Baby chicks

[Sundown Feathers]

Have 3 little girls that are 18 weeks old. They have not lad anything as yet. If I decided to get a couple of newborns how long would I keep the older chicks away from the babies and will the older birds hurt the new ones.

 

[nuthatched]

18 weeks is a little young to lay, especially since it's been winter for a majority of that.
You can brood in the coop but yes, it's best to keep them separate until the babies are big enough to hold their own, so everyone gets used to each other and so the big ones don't kill the littles.

 

[NatJ]

Have 3 little girls that are 18 weeks old. They have not lad anything as yet. If I decided to get a couple of newborns how long would I keep the older chicks away from the babies and will the older birds hurt the new ones.

If you just put in little chicks with no introduction period, yes the big ones will probably hurt the little ones. They may peck them to try and figure out what those little things are, or they may recognize them as chickens but try to chase them away because they are strangers, or they may squish them to stepping on them or crowding them.

One solution is to raise them separately until the younger ones are feathered out, then have them live in side-by-side pens (separated by wire mesh) until the younger ones are the same size as the older ones, then try putting them together. At that point, they may be used to each other, or they may fight over the pecking order, but the young ones are big enough and tough enough that they are not very likely to get killed.

But some people have good results integrating young chicks into an existing flock, without trying to raise the young chicks all the way to adult size first. Here is an article with some ineresting ideas on that way of doing things:
https://www.backyardchickens.com/articles/my-coop-brooder-and-integration.74591/