From what year should a penny be worth more than 500 dollars?
I assume you're referring to a U.S. cent and not a British penny, since the terms are often confused.
The value of any coin depends on its date, condition, and mint mark just for starters. There are a lot of other more subtle variations but those are usually the big three. The U.S. has struck cents since 1793 at Philadelphia, Denver, and San Francisco so you have a huge number of possibilities to choose from. As examples, here are links to cent values at Numismedia, just one of many sites providing numismatic price guides. I scanned through them and found many dozens of coins meeting your criterion.
Large cents: http://www.numismedia.com/fmv/prices/lrgcnt/pricesgd.shtml
Flying Eagle: http://www.numismedia.com/fmv/prices/flycnt/pricesgd.shtml
Indian Head: http://www.numismedia.com/fmv/prices/indcnt/pricesgd.shtml
1914-D in at least AU grade
1922 NO D in at least VG grade
1955/55 doubled die obverse
These are the 4 main coins that would most likely reach the $500 price tag.
All wheat pennies, regardless of year are worth at least 3 cents if the date is readable. Some wheat pennies though, are worth quite a bit more. For instance, the 1909 S VDB penny is worth several hundred dollars even in worn condition and so is the 1914 D penny. However, a 1909 S VDB penny in pristine condition could be worth thousands of dollars. In general, the older the wheat penny the more valuable…