It is a personal decision. I take biosecurity seriously enough that the only way I add to my flock is through hatching eggs or chicks from a hatchery. Different people take different approaches to this.
Some things to understand about quarantine. Mrs. K is right. True quarantine needs distance, not just a fence. Different diseases are spread different ways. Some are passed on by eating each others poop, some from sharing food or water dishes, and some are spread by the air. People who stick them in the coop in a cage or dog crate are not really quarantining.
Some flocks have diseases that they are immune to. Coccidiosis is a great example, but there are plenty of others. The will never show symptoms no matter how long you quarantine them, but as soon as you mix them with your flock, they flock is infected. And it is just as likely that your flock will infect the newcomers with one of these.
I personally don't see mites, lice, or worms as a great threat to your flock from newcomers. These are more inconveniences than risks compared to many other potential things. I do think it is a good idea to treat for lice and mites, plus worm any newcomers before you mix them, but these things are not going to kill your chickens as long as you look out for them.
To me, a serious quarantine not only includes housing them truly separately, but to not carry the food and water to then in the same containers. Don't wear the same shoes when you go from flock to flock but have dedicated shoes for each flock. I would not go so far as to change clothes and shower when going between flocks, but the commercial operations do that. They even require delivery trucks to disinfect their tires before dropping off feed or other supplies. I personally would not buy a separate lawn mower for each area. I know I'm being a little ridiculous with that, but I want to show how serious the big commercial operations take biosecurity.
As I said, I don't add other older chickens, but to me a real quarantine would include putting one of my existing flock with the newcomers to see if they are hiding something they are immune to.
Don't get me wrong. I thin quarantine is a great idea if you understand what you are doing. Chickens from closed flocks, which means that they are not exposed to strange chickens, are not likely to show anything when in quarantine, especially if the previous owner would recognize a disease instead of thinking sneezing is normal. But chickens that come from chicken swaps or new chickens have been added to the flock recently are certainly at a much higher risk. Quarantine is an extremely valuable tool but you need to know how to use the tool for it to be effective.
