It won’t decide what’s fair it’ll tell you what’s true.

How can someone possibly know they are currently underpaid without evidence that they can earn more than they are currently earning?

The evidence is tell me how many tech companies, especially startups, have a large ratio of over 40s? If age discrimination were better enforced, the OP might have a very different (and more fair) market to try your experiment. Until that happens, the OPs observation that they are under compensated vs. other workers of a similar skill set would be where I'd put my money.