I'm going to guess that it all depends on the personalities of the animals involved and on exactly how you've got them set up as to living space. Nervous, easily-startled chickens and rambunctious lambs of either sex, confined to something the size of a horse stall maybe wouldn't mix too well. Placid pullets and a mild-mannered, quiet sheep with half a barn floor to meander about in, on the other hand...well, you see my point already, I'm sure.
20 weeks can still be too early.
My red sex link laid at 20-21 weeks but it took longer for my SL Wyandottes and my Naked Necks.
They were closer to 22-24 weeks.
They should have red comb and wattles and be checking the nest boxes out quite a bit.
This behavior can last a couple of weeks before they start laying but if they’re doing it , it’ll be soon.