Find when device was activated
If the date of activation matches the date of adding your Google account to the device, there's an easy way to check (unless you did a factory-reset in the meantime, and had to re-add the account):


with your Google account's credentials, log in to the Google Dashboard
Scroll to the section labeled "Android" and expand it (by clicking its title / the LGM [little green man])
Check the entry for the device you want to know the date for


At this place you should find the date you registered your device. That doesn't reflect the first time a device was added, but the last time the Google account was activated on the device. So if you activate your Google account following a factory-reset, this field is updated and thus reflects this point-in-time.