Hey there, I'm not sure I understand what you mean by "first activated". In general, you activate a SIM card, not a phone. This would be associated with your current phone plan, not the device itself. Your carrier would be able to provide that info. If you're referring to when your phone was first purchased/turned on, then most folks tend to add their Google account during setup, which might be why there's a suggestion to check your Google account to see when the device was added.

The IMEI is potentially useful as it's a device identifier, but generally doesn't matter to anyone except your carrier.

Start with why you need to know this?

You could also check your emails for one of those "Welcome to your new [insert device name here]" emails, or for a Google sign in notification for a new device on your account.

Maybe there are update logs? Not sure if those persist after a factory reset though