Let's set some parameters for this to make it easier to answer:
- riding for more than 30 years.
- riding for more than 5 years with a power meter to track the various CPx values.
- older than 50 years (which seems to be considered as "turning" point, i.e., you are unlikely to get faster/stronger after that age).
To avoid getting stuck at some plateau the training approach might have been changed a few times, e.g.,
- "hard" rides, e.g., riding close to CP60 for 60m.
- sweet spot training: 2x20m at ~95 FTP.
- 1x20m at CP20, rest easy.
- HIIT
- long rides (3-4 hours) below Z3, plus some intervals.
Let's say CP20 is about 4.5W/kg and has barely changed over the last 5 years.
How do you know you could (not) reach a higher value, e.g., 5W/kg?