Totally agree.You can but it depends on the size of the fry. If the older fry are big enough to eat the smaller fry, then no. If they are around the same size then you can. I have put different aged guppy fry together. I have even mixed guppy, swordtail, and molly fry together who were different ages (the guppies were older though). Is the tank planted? If so, then if they are around the same size, I would say go for it.