The first two numbers of a serial number identify the year a banknote was manufactured. For example, a $50 banknote with a serial number DB 99 572038 indicates the banknote was manufactured in 1999. Whereas a $50 banknote with a serial number DB 03 572038 indicates the banknote was manufactured in 2003.
Polymer banknotes last longer than paper banknotes because of the non-porous, non-fibrous nature of polymer, and the overcoating of polymer banknotes with a clear varnish. As a result, polymer banknotes do not absorb moisture, nor do they stain or accumulate dirt as easily as paper banknotes. The life of banknotes also varies by denomination. Low denomination banknotes, handled more frequently and roughly in day-to-day transactions, do not last as long as higher denominations.
In the course of being used, banknotes are subjected to quite harsh treatment. For example, banknotes are often folded, crumpled, exposed to moisture or heat, stapled and so on. This ultimately leads to signs of wear such as holes, tears and ink wear. When banknotes are no longer fit for further use they are replaced with new banknotes using strict criteria that the Reserve Bank has set to maintain the high quality of banknotes in circulation.
For more information, read the Bulletin article – The Life of Australian Banknotes