What limits the lifespan of helicopters?
Helicopters are also subject to metal fatigue and there have been accidents attributed to it but the primary determination is component life times.  All of the engine, transmission and rotor parts are lifed and most of the control mechanisms.  Depending on the helicopter, there will be many more lifed components.

Some helicopters also have fixed intervals between major overhauls by the manufacturer.  For example, the R22 is effectively rebuilt to an "as new" standard by Robinson at 2,200 hours.

Corrosion might also play a part.  There may be a time when it is just not economical to continue to repair and service the airframe.