I want to know when this device was bought

Hi beenyleena,

Welcome to the Apple Community.

Wondering if it would help to search your e-mail for "Apple" e=mails.

and find the e-mailed receipt that Apple usually sends after a purchase.

OR

You may have kept a paper receipt in the original packaging, as some of us do.

So if you find either you will have both purchase date and serial number.

Or

Apple support have an article:

Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

And / Or

If you have the device's serial number, you can check Your Service and Support Coverage,

(likely revealing the purchase date, and / or the warranty coverage period / expiry of the device).

https://checkcoverage.apple.com/au/en/;jsessionid=node0525iculj19xds7x41izqlawf3077853.node0

All the best :-)