You're missing the point. The 20 year guy probably lives in base and doesn't want to fly as much. I don't want to be a commuter, sitting in a crash pad, with minimum days off, even if I fly couple hundred hrs a year. It doesn't matter if i've been there 10 or 20 years. If your company has good reserve rules such as no ready reserve, many restrictions what crew scheduling can and can't do, reserve will go more senior. At my company / base. If you take the top half of the FO, 25% bid reserve, and the percentage is even higher for CA.