Basicly you can check how long has your harddisk been powered on with the command below

The sample below shows the harddisk has been powered on for 9577 hours which you can calculate to 399 days. Which also means that the harddisk is more then a year old.

# smartctl -a /dev/sda | grep Power_On
  9 Power_On_Hours          0x0032   090   090   000    Old_age   Always       -       9577