I feel like everybody is reading what I'm trying to say backwards.
Based on the leaked 1G -> 5G timeline, Lois isn't just 3-5 years older. She's potentially 70 years old based on Clark's first appearance as Superboy in the 50's.
I DON'T want Lois made irrelevant and eventually killed because her human biology doesn't allow her to keep up with Clark's slower-ageing biology.
I DO want her around, and I DO want her to continue to be an active part of the mythos. That's why I was trying to come up with a scenario that DOES keep her 40-ish where she can be a vital player, not 70-ish where she will potentially be rarely seen or shunted off into rarely-seen land.