$ for i in /dev/disk/by-id/ata-WD*; do sudo smartctl --all $i | grep Power_On_Hours; done
9 Power_On_Hours 0x0032 030 030 000 Old_age Always - 51534
9 Power_On_Hours 0x0032 033 033 000 Old_age Always - 49499
One has a total powered-on time of 51534 hours, and the other 49499 hours.
As for their actual age (manufacturing date), the only way to know is to look at the sticker on the drive, or find the invoice, can’t tell you right now.