Unfortunately, it was only the previous film where Kahn declared it was "15 years" since he was marooned.
"20 years" might refer to another significant rebuild, (turning Pike's Ent into Kirk's) but despite the changes they would pale compared the TMP refit!
Perhaps Kirk's trip to the galaxy edge went down really well with Earth media and Starfleet began using the Enterprise as her "named" ship for PR purposes. 20 years later, they're ready to move onto the Excelsior.
So, what the Admiral really meant to say was "The Enterprise as a publicity device is 20 years old - we feel her day is over"
Or something else - anyone?