I've noticed that AI aircraft on approach only extend the landing gear when the aircraft is about 500 feet from landing. Is it possible to change this so that the gear is extended about 4 or 5nm out, to make it more realistic, or is it something which is a feature of MSFS and can't be amended ??

Which aircraft type ? Whose AI model? IFR or VFR flight plan?Which airport, or all airports?By and large in my experience I've never seen that behavior except in VFR aircraft generated in the first five minutes or so of starting FS.And we all know anything which happens in the first five minutes is not normal - takes at least 15 minutes for the AI world to stabilize.