They actually aren't. I know I'm probably going to get preachy about this so I'll only say it once or twice!
Reusable mugs are only to be used for the year of the trip you bought them on. Period.
no, you won't be stopped if you are using old mugs. However, you are technically stealing from WDW by doing this. There are some oolllldddddd mugs that were bought when they were still advertised as "good for life" but they now ask that you buy a new one each year. It's completely up to you, like anything else is! Personally my mom and I choose to buy new mugs each trip. It's also fun because they are frequently different year to year and it makes for a fun collection :)