Old houses have a multitude of window and door types. We have one overarching piece of advice for anyone with an old house.  

External joinery will only deteriorate if it gets wet.  And stays wet.  Windows and doors can last for hundreds of years if they can breathe, and moisture isn't trapped into them. In the old days, timber was often painted - but with breathable paints - linseed based in the main.  Modern joinery is always painted with 'micro porous', 'breathable' paints - all of which are made with varying amounts of plastic emulsion - which is definitely NOT breathable, and traps water - thus rotting the timber.  It's why paint flakes - the timber behind it gets wet - tries to dry out - blows the paint off, and starts to dry. Window and door frames are usually fitted with all manner of expanding foams and silicone / acrylic sealants these days. These trap moisture into the frame, and it rots. You cannot use foam or silicone where it is in contact with timber - it is guaranteed to rot. I don't know of a single 'sealant' that comes in a tube, that will work - they are all impermeable, and all rot timber. 

We use oakum, which is used to seal the deck planking of boats, to seal around doors and windows. It is twisted into a rope, and hammered tight - then pointed over with a little bit of hot lime putty to seal it, and stop birds from pecking at it.  In the photos below, you'll see Dez installing new oak windows in a Shropshire Farmhouse - and twisting the oakum using a bent nail and drill - this makes a rope, which is then hammered into the gap to produce a tight seal, very flexible, and waterproof - but at the same time breathable, so water isnt held against the timber.