Hi A. - there isnt an age limit on cribs because they are furniture. The 6-yr rule is for car seats. If your crib was new or near new 5 yrs ago, someone will be able to get years of enjoyment from it.
The crib we used for both of our boys belonged to my sister who used it for her son born in 1996. As long as it is sturdy, does not have the possibility of lead based paint (paint prior to 1977) and the slats are 2 3/8' apart as one other post suggested, it should be a great crib.
You also want to be cautious that it does not have any finials on the corners or posts on the corners that could catch clothes or toys and be a strangulation hazard.
It would also probably be a good idea to check for recalls on your crib before you sell it.
good luck to you!