What is the rule about using either 'ser' or 'estar'?
Soy andando hoy, o estoy andando hoy? Soy un hombre de vientecinco anos, o estoy un hombre de vientecinco anos?
Which are correct?
While the temporary vs. permanent "rule" can work at the beginning of learning the differences between "ser" and "estar", it soon reveals itself to be flawed. Though in many cases "estar" is used in situations that "will change in a few minutes, hours, days or weeks", in many other cases this is not true.
The classic example to prove this point is the use of "Estoy muerto". Being dead is not going to be something that changes, so trying to apply the "temporary rule" doesn't work here.
And "ser" isn't always used for "permanent situations". For example, one's profession is not a permanent situation - you get fired, quit, or retire at any moment. Yet, "ser" is used with professions.
Basically, while ChmacoMalo has provided a good start to understanding "ser" vs. "estar", I warn you against using the temporary vs. permanent rule.
Here is an excellent reference article that goes more in depth to what I was talking about:
Soy andando hoy, o estoy andando hoy?
Estar + present participle (gerundio) forms the progressive tenses
Soy un hombre de veinticinco años, o estoy un hombre de vientecinco anos?
Ser is used to classify, identify or define.
a man of 25 classifies or identifies (distinguishes) you from men of other ages.
Hoy, ESTAS andando. It's something you a doing, a verb, that will change. Now you esta walking. Later, you might be esta in the bathroom. Tonight, you esta at the movies. You ARE (Ser) a man, will be a man, haste forever, or you get a sex change, you HAVE (tener) 25 years. That's how that whole ser/estar thing words. Ser is what a thing is and will continue to be, Estar is a conditional thing, that will change in a few minutes, hours, days or weeks. I'm estoy on the sofa right now. Yo soy African american. Tomorrow, I will estoy at work, I will still be (soy) an african american. 50 years from now (God willing) I will estoy retired, but I will still (soy) be an african american. Por claridad si no te has visto, ESTAR is a place. Yo ESTOY in mi sala. Tu estas in tu casa. Ella esta en el bayo. Ellos SON (ser) flacitos y ESTAN (estar) a la fiesta.