i had 2 hens , one died, I need a new one, what age is safe

[aart]

safest bet would be newly hatched chicks from an established hatchery

Ditto Dat!!

I've found that adding little chicks is much easier than older/bigger birds, but you do have to be setup well for it to work:
https://www.backyardchickens.com/articles/integrating-new-birds-at-4-weeks-old.72603/