It is recommended that chicks should be more or less fully grown before being introduced to a new flock, but depending on the flock and the number of chicks introduced, you may get away with doing it sooner, say when they are 2-3 months old. I've introduced chicks to my flock at fairly young ages in the past, but at the time I had quite a large area for them to roam and the young ones managed to stay out of the older chickens' way most of the time.
To make introductions at any age easier, when the chicks are old enough to live outside full time, keep them in the same space (coop and/or run) as the existing flock for a few weeks, so the established flock can get to know them and get used to seeing them without being able to physically get to them. When you decide to let them mingle let them them free range together, if possible. There will most likely be some pecking and bullying, but unless it's excessive and/or blood is drown don't intervene. The flock will need to re-establish their pecking order and this is how many of them do it. They should settle down within a few days though.