Not sure I get your logic there Sam.
I don't get to choose when I fly as an FI either. I put my availability into a book and if someone books it, then if the conditions are OK, I go flying.
If an airline pays me to fly within the FTL scheme of the company and the conditions are good enough and the a/c is serviceable, then I go flying, but that's the same with any job. Instructing, air taxi or airline.
If you were a full-time FI, then you have to have a reason for not going up, not just because you don't feel like it. The same as if you are working for an airline.
Instructing and airline flying are different, but they aren't that far removed. People who are good at one, are often good at both.
Age isn't really relevant in this discussion as you're 27 and I'm a crusty old 28!