Generally they say 15-23, I believe. But for the one in Pittsburgh, I don't adhere to a strict limit. Any high school players are encouraged to come out, and there have been a few 8th graders that have come out and done very well in the past. So it depends on the individual.
On the other side of the spectrum, as long as you are serious about coming out, I certainly wouldn't send anyone home. With that said, it is our job is to find players to help us win the NL east. So if someone is on the older end of the spectrum, I would have some expectations that they were more advanced in their abilities.