At the moment, I'm integrating a two -year old abandoned hen that I adopted into an existing flock of twenty chickens of a wide variety of ages. This is day four and it's been extremely hard on this little new-comer.
It will prove not much different for your single older hen. She will be vastly outnumbered and a stranger to the flock of three-month olds, who are an established social unit. They won't likely be welcoming, and more likely will see the necessity of putting the alien hen in her place by chasing her and pecking her savagely on the rump to drive home the point.
It's a lot of bother inserting a strange adult chicken into any flock, and especially hard on the lone chicken. By all means keep your hen confined safely so she isn't unnecessarily bullied and hurt. I found that a cage or crate to house the lone chicken in the midst of the flock isn't adequate to keep the chicken safe from harm. My little adoptee managed to get her comb ravaged right through the wires of the cage so I moved her into a safe pen where she has room to retreat from the little savages when they crowd the fence separating her from them.
Day by day, though, the flock is gradually losing interest in the stranger among them, and in a few weeks, they should all get along pretty well. Meanwhile, I supervise periods where the new hen is put in among the flock so she can learn whom to trust and whom she needs to maintain a distance from. Then I put her back into her safe pen to rest up for the next session a few hours later. If it's a nice day, they all free range together and that's a remarkably good way for everyone in the flock to learn to relax and get along.