Each month the shield on your userlookup will change (if your account is less than a month old it changes every week). The shield displays the age of your account. In other words: how long you have been playing Neopets. Some of you might be curious to see what shield you can expect to see on your lookup in the near feature. This page contains a list of all shields that have been made so far, they go up to nine and a half years.
The shield on your lookup might be a little off from the real time you have been playing Neopets, but Neopian Times Editorial #197 explains why:
Hooray new shields :o, but why only every 6 months?
Neo finally decided to release some new shields after the 6 year mark. Instead of them being every month though they are now every 1/2 year.