NOK air aging fleet?

Nok Air seems to be having some maintenance problems with their ATR aircraft.
After sending an inquiry regarding their aircraft I received a typical corporate response:

Refer to booking number xxxxxxx
As your complaint, we are so sorry to hear that you got an inconvenience during travel with Nok Air due to our flight DD9403, DD9406 on 30 January 2013 is suddenly delayed with operations reason. We recognize that time is valuable for passengers, however the most important for Nok Airlines are security and passengers’ life, we concern that it is our responsibility to take all passenger to the destination with safety. We hope you will understand on this matter, and enjoy your flight.

Again, please accept our sincerest apologies for any trouble or inconvenience we have caused you. We highly appreciate your feedback as it will assist us in becoming better at what we do.

We look forward to serve you again in the future.

Sunday morning, half an hour before my flight, I noticed a jet heading south flying over the Sakon Nakhon Airport. 
Half hour later, my flight arrived.  Obviously not the ATR, but a Boeing 737.

No complaint from me regarding the replacement aircraft. Most of the flight we stayed inside clouds. A bit bumpy at times.
Not too bad. The landing was a bit unnerving. When the reverse thrusters were activated, the whole plane rattled so badly, I was expecting the overhead baggage compartments to come crashing down.