Younger and Older are relative terms. And you really have to make good judgement calls on this one. I have raised chicks that I'd purchased within two weeks of each other all together and it has turned out fine. But my personal judgement is that I wouldn't widen that spread any further than the two weeks. I also wouldn't put a single 2 day old chick with a group of chicks that are 2 weeks old. Only in equal numbers so they can band together. If you're talking about chickens past the 3 month mark and fully feathered, you always introduce them carefully and in pairs at the least. Introducing a single chicken to any established flock is asking for it to be bullied and probably injured. You really just need to judge your current flock and what you want to introduce; you will know your chickens and what they will accept, and what they will not. All that being said, past that 6 month mark or so, chickens are chickens to each other, so a 4 year old chicken will meet a 6 month chicken and it's just another chicken. Most chicken owners add to their flock yearly, at least as far as I've seen around here!!! (chicken obsessed much???) And they do so knowing what their flock will accept and on a time scale; for example, when introducing my new chicks to the flock, they live in a separate run and coop for the first month or so, so the chickens can see each other, but not fight. Hope this gives you some semblance of an answer to that question.