How do I find the age of my Apple Watch?

There is no way to find the exact age of a device. You can determine when it was released and calculate age based on that, or use your receipt /invoice and use the date there to calculate it.

If you want to know what model / series / generation it may be, you can enter your serial number at Check Your Service and Support Coverage - Apple Support and it will tell you.

You can also find the model, series and generation here: link-> Identify your Apple Watch - Apple Support