I wasn't think of 'reflexes', more the aquisition of complex motor skills late in life. Any newly-qualified PPL will still be 'flying by numbers' to a certain extent (just past the motorway throttle back to such and such RPM, lower flap, re-trim to such and such speed.... that sort of thing) and conciously thinking about which flying controls to move and how much to move them, a bit like a novice driver thinking about pressing the clutch to change gear.
After a while, such things become automatic; flying the aeroplane around the circuit requires little or no concious thought about moving the controls - you just do it! You don't think about pressing left rudder with power, or right as you take power off, or constantly chasing heading and speed - the needles wander far less as one gets experienced... etc. This means that if something goes wrong (engine failure, say) one can devote most of one's brainpower to planning and executing the forced landing since actually flying the aeroplane will be or should be semi-automatic.
Will someone learning to fly late in life be able to do this within a reasonable time.... or at all?