Technically, an antique is defined by the US Customs Office as a collectible object being over 100 years old. By that measure, not many of the signs you’ll ever see will be real antiques. However, most people will refer to anything old and collectible as an antique in common speech. It’s really not important to be picky about what’s an “antique” and what’s not because with these porcelain enamel signs the value is usually not based on the age as much as the imagery, brand, condition and rarity.
Even if age was an especially important determining factor it would still be very difficult because unless there are obvious features like a date on the sign there are not very many options for dating most signs. In the case of some companies you can get an age range based off of the particular features of a logo or trademark. You can also sometimes get a date range if you can determine what company manufactured the sign, but even then you get a range of decades in many cases.
I have a lot of people send me contacts concerned primarily with the age of their sign. The age really isn’t that important for the value compared to condition and demand from collectors. In general, there’s a large market out there for petroliana signs for example that tends to keep prices high for signs related to oil and gas stations so even signs from the 50s and 60s regularly sell for over $1000 while signs of similar condition and age from different subject matter might sell for half as much.