IMO all data can be useful but the variables skew the data and that has to be kept in consideration when evaluating.
I'm 68 and currently don't do any structured FTP tests though my FTP has gone up and I've achieved new FTP's data during some Zwift races/events this off season.
Like all data, your body's input may change it due to illness, fatigue, etc. and has to be taken into consideration when using this data for fitness tracking/improvement, etc.
I think they are useful tools but you have to understand the data, its' source, etc. in order to use it to improve, etc.