22 years old for a B733 may seem "old" but its not unusual. The remaining Monarch B757s are 22/23 years old-their A300s are around 21 years old. The B744 fleets of BA,KLM & Lufthansa are, in most cases over 20 years old. It will be a few years too before these will be replaced.
Subject to cycles, hours & MX costs, the LS 733s could go on for some time yet. The European B732s that used to knock about were over 30 years old.