>So then the question is why didn't I get the xp this time?

You were outside the level requirement for that particular area. They don't always exactly match up with the 'areas' command.

>Also, if I go back and do it all over again later, will I get
>the xp?

Yes if you are within the correct level requirement.

> How do I know when it's ok to do that?

Try it, discuss with the guildmaster, if you don't get xp then you know you are not the right level. (note that you can be too high or too low for specific areas - although too low generally is limited to area explores).