Age has nothing to do with safety (excepting the pilot of course).
The S61n which all passengers love is the oldest of the North sea aircraft. The Pumas, which the complaints are about, are getting on now (some of them are 20 years old) but are as safe now as they were when new. The amount of maintenance which goes into them is incredible. Apart from the routine daily/weekly/monthly/hourly servicing they get, they get stripped down to the bare metal every 7000 flying hours (about every 5 years), are inspected very closely and completely rebuilt. The aircraft comes out like new.
We know every nut and bolt on them and are very comfortable about their reliability. What has changed over the years is the increased regulation and safety culture. There are more precautionary "returns to base" due to minor defects which previously the pilots may have continued with and had fixed when the aircraft finished the flight.
The Pumas probably do need replaced but not for safety reasons. They are getting heavier as all aircraft do with age and don't carry as much payload. There are aircraft becoming available which carry more payload per pound of money. The downside of new helicopters is that there is a big learning curve for the first few years and the number of precautionary "returns to base" or unserviceabilities on start up may be higher for a while compared to the older aircraft.