How do i find out how old is my ipad?

There is no way to identify a precise date of manufacture - this information being known only to Apple. Device serial numbers are an unreliable indicator.

Starting with knowledge of the iPad model, you will be able to identify the model year. Your purchase invoice, assuming you purchased your iPad from new, will provide a more accurate indication.

You can check your warranty status online with Apple. While your warranty may have expired, if will likely have commenced around the time of initial device activation:

Check Your Service and Support Coverage - Apple Support