People can start and find wonderful and fulfilling lives in their 40's. Personally I have no idea what the OP is like and if this discussion is the only interaction you've had with the person, neither likely do you. Which means both of us are unable to guess what he can still achieve in life. A borderline depressed person will describe their circumstances and capabilities in a way that plausibly does not in fact reflect their full potential.

There are some career paths for which not only doors close at an early age but they require specific circumstances to even be feasible - like an Olympic gold medalist. Most lives are not like that (than heavens) and people are able to find meaning and fulfillment in lots of circumstances.

If you don't know a person, then the charitable interpretation is to trust they are capable of humanly achieving what ever they set their minds to, that people starting from similar circumstances have achieved. With the input data (40+, masters) the range of possibilities is quite large.

"Faking it till you make it" is perhaps not an accurate description. It presumes you need to prove something to someone. A person just needs to find the thing they like doing, and then thrive in doing that. Once you thrive people will notice anyway. There is nothing wrong in starting humbly, and then moving upwards once skill and confidence builds up.