This is my first time with chicks, but my six were all different ages when I brought them home. Two were almost two weeks old and two were newly hatched (the AM I picked them up). They were all fine. The only "problem" I had was the youngest chicks (two silkies) tried to sleep under the largest two (which are large breeds)! I have had them for about 6 weeks now and despite age and size difference, they get along great!
Also, based on what I had read I made sure they had a lot of room in the brooder and I used a red light which is supposed to help.
Good luck!