Steven is right, sometimes when to use estar and when to use ser can be a bit fuzzy. In fact, occasionally the Peruvian girl that I study with once a week has to think a bit which is the most appropriate verb to use. Same thing with por and para. She knows instinctively which to use, but has to think why at times.
I have had better luck thinking about estar and ser as a condition that is subject to change (estar) vs. an essential characteristic, what something is (ser.) Having said that, I know that the person will not always be young, but things that define what a person is in relatively long range are described by ser. More fluid descriptions likely to change more quickly are described by estar: estoy cansado...I am tired, but hopefully that condition will change once I have rested.
In fact, I just realized that the word solid is a good way to think about ser, and fluid a good way to think about estar.