For story reasons, I'd like one planet habitable, plausible, but in some noticeable way older than Earth. Just not all planets are 4,5 bilion years. Tidally locked planet that orbit red dwarf, saves me from problem of stellar evolution and shifting habitable zones.
1) I should give it less fissile isotopes, there would be clearly less U-235 in relation to U-238 what would affect local nuclear sector.
2) Less radioactive isotopes -> colder core -> weaker magnetic field.
3) colder core -> less active surface, avoid any really high, recently formed mountains
4) Adjust for atmospheric loss, which can be measurable by assessing ration of hydrogen and deuterium
What else should I realistically adjust? How old I can make this planet not to have to worry about early metal-poor universe?