There's a sticker on the box that tells you the manufacturing date of the kit - rule of thumb is to try and make your wine within one year of this date. I saw a post where someone had a kit that was several years old. They added fresh yeast and were on their way.
Look for a date code on the wine kit box label. That code represents the date that the kit was put together by the manufacturer. Once you have the code, do a search on this site under the search term "date code". There will be links to instructions on how to decipher the date codes for various wine kit manufacturers. The rule of thumb is that the kit is current up to a year from the manufacture date. The viability of kits older than that depends on the environment where the kit was stored.
If you have a wine kit older than a year, do not discard it just out of concern for its age without trying to make the wine first. Just replace the yeast packet with a new one of your choice and give it a go. Taste it when racking from the primary fermenter into the carboy. If it taste a little bit sweet and fruity with no off flavors, you're probably fine.
While I'm not saying that a wine kit cannot go bad with time, I personally haven't heard of anyone mentioning such an event locally or on the Internet.
I hope that you make the kit. Be sure to keep us informed with your progress. Your experience will benefit others who have this same question in the future.
Agree with what was posted before, but want to add that almost all yeast packs have an expiration date on them. Usually along the outer edge of the pack and often hidden by ridges. They are a pain to read, but it should be there.