The "age" of an airframe is measured in 1. Pressurization cycles, and 2. Hours in flight...And the Boeing 737 of today has little in common with the version from 1967 when it first flew, much like comparing a '67 Corvette with a 2012...
Each time a frame reaches a certain number of cycles it'll go through some form of check:
A Check — This is performed approximately every month. This check is usually done overnight at an airport gate. The actual occurrence of this check varies by aircraft type, the cycle count (takeoff and landing is considered an aircraft "cycle"), or the number of hours flown since the last check. The occurrence can be delayed by the airline if certain predetermined conditions are met.
B Check — This is performed approximately every 3 months. This check is also usually done overnight at an airport gate. A similar occurrence schedule applies to the B check as to the A check.
C Check — This is performed approximately every 12-18 months. This maintenance check puts the aircraft out of service and requires plenty of space - usually at a hangar at a maintenance base. The schedule of occurrence has many factors and components as has been described, and thus varies by aircraft category and type.
D Check — This is the heaviest check for the airplane. This check occurs approximately every 4-5 years. This is the check that, more or less, takes the entire airplane apart for inspection. This requires even more space and time than all other checks, and must be performed at a maintenance base.