The age that a vehicle has to be to be considered an antique depends on the state as some states may require the vehicle to be 30 years old or older to be an antique.
And some other states will allow you to antique tag a vehicle after 25 years old so it would be best to check with the state you're considering an antique tag in.
If you told us what state you're tagging the vehicle in as an antique we could give you an exact answer but usually if the vehicle is at least 25 years old or older it can be antique tagged.
However be aware that when you do antique tag the vehicle you cannot drive it to anywhere you wish or use the vehicle as a daily driver vehicle.
But when you antique tag the vehicle you can only drive it to and from auto shows and events or to the gasoline station.
So sometimes an antique tag is not all that great even though you don't have to renew the antique tag each year.