How old does an ISA Brown hen have to be to run with roosters?

[aart]

the older rooster is an ISA Brown, and he is quite big is that still okay?

Size doesn't matter, demeanor and technique does.
A good cock/erel, no matter his size or age, will not try to mate a female that is not in, or close, to lay.
There's no telling, form what little info we have, how the 2 males will act around the hens and new pullets. Multiple males can often create chaos due to competition.

Were these two males living with the 3 hens before the pullets came along?
How do the males get along, do they fight or over-mate then hens?
Your 150sqft run is just big enough for 15 birds.
Wonders how big your coop is.

 

[Free Rrange Farmer]

Size doesn't matter, demeanor and technique does.
A good cock/erel, no matter his size or age, will not try to mate a female that is not in, or close, to lay.
There's no telling, form what little info we have, how the 2 males will act around the hens and new pullets. Multiple males can often create chaos due to competition.

Were these two males living with the 3 hens before the pullets came along?
How do the males get along, do they fight or over-mate then hens?
Your 150sqft run is just big enough for 15 birds.
Wonders how big your coop is.

Yes, the roosters were living with the girls but now they are separated from each other and the hens. I did have suspicions of fighting but no real evidence. Another reason why I separated the hens and roos was one of my hens had a gash on her butt. My older rooster mates with a hen that is 5 and doesn't lay.

 

[aart]

Yes, the roosters were living with the girls but now they are separated from each other and the hens. I did have suspicions of fighting but no real evidence. Another reason why I separated the hens and roos was one of my hens had a gash on her butt. My older rooster mates with a hen that is 5 and doesn't lay.

I would choose one and get rid of the other(if you want flock chicks in the future)......
...depends on your goals.