Hi and welcome :-)
Basically the questions stay on site, and they are not removed. You can answer questions as long as you feel you have something to say, even if the questions is more than a year old (and of course younger questions as well).
The decision whether to close a question lies entirely in the asker's hands, and it is up to her to decide if she had found the answer she was looking for or not.
I hope this answers your question. Please feel free to address me directly here or by email if you have any more questions:
neta.silberg@yedda.com
thanks :-)