They all have to be recertified 12 years after initial manufacture. There are several methods of recertification, the most common being visual recertification, good for five more years. A visually recertified bottle will have a new date stamped in the collar preceded by the letter "E". (Think of "E" as standing for "Eye," hence visual recertification.) There is no limit to the number of times that a bottle can be visually recertified, as long as the bottle is in good condition.
If you've got an aluminum bottle in good shape, get it recertified! It's a BUNCH cheaper than buying a new one!! (Call around for prices. Recertification of bottles, like pricing of propane, is completely unregulated: Places will charge whatever they think the market will bear.)
Oh, and while you're at it, check to see that your bottle has the requisite OPD valve (Overfill Prevention Device, which you can see by the non-removable, triangular shaped handle on the top). If it doesn't, then have them install one on your bottle at the time of recertification.