Cost of living is very complicated. Most software engineers making $100,000 are young, single, don't have kids, and are comfortable with renting. Most software engineers making $200,000 are older and have kids, which means they need larger living spaces and have a strong preference for owning rather than renting. On this alone, you need different conversion factors. Also, the distance/price curve is different for different cities. Manhattan is very expensive but there are affordable neighborhoods 30 minutes away; the Bay Area isn't as expensive at its core but the catastrophic prices cover a larger area.
Unless the salary is commensurate with increased value or output, I see no reason why older SDEs should make more than younger devs. To be clear, there is often great value in older SDEs, but "because they have kids and want a house" is not a reason to pay them more.
