I would say not more than 2 weeks apart in age. So, if you want to get more chicks today, you could probably mix the original chicks and the new chicks after a couple of days. However, if you wait another week and then get more day-old chicks, I wouldn't put them together immediately. I guess it depends on how soon you get more chicks.
Right now, I have a group of 9 two-week old chicks and 13 3-day old chicks. I won't put them together right now because of the size difference. The just-hatched chicks are still popping along unsteadily and falling asleep standing up. I do think that in about 2 weeks, though, they could be combined as the difference between them won't be so noticeable anymore. 2 1/2 week old chicks and 4 week old chicks will probably be okay together so long as there's adequate food and water access for all.