How old is my iPad? Model number is MM192LL/A

You have an iPad Pro which was originally sold sometime between March 21, 2016 and June 5, 2017.

(155860)

No way to know exactly how old it is.

That model of iPad, is a 9.7" Rose Gold, 128GB iPad Pro.

9.7" iPad Pros were released in March of 2016, and were sold until June of this year.

That means it can be anywhere from a 20 months old to only 6 months old depending on when it was bought.

Did you buy it new?