How to decide if an old wood door is worth painting?
Is is too late?

No, I don't think so

What should I consider?


Does the door still operate or is it merely decorative?


i.e. will repairs to the mechanism be necessary? You may need to grease, repair or replace the wheels up top and/or fix up the rail it hangs from.
Or are you happy with it stuck open and you just want it to look nicer different?

There is some rot at the bottom, particularly on the right. Is that sturdy enough or does it need to be repaired to keep it from falling apart in use?


Do you have the skills necessary to repair the door if "freshening" it includes replacing the rotted wood?
Repair could involve replacing an entire board or just cutting off the rot and patching in a new piece.
Do you have extra old wood laying around that would match in width, thickness and general patina? Modern dimensional lumber probably will not match, especially in thickness. 
Any new replacement lumber will look different, even if the whole thing is painted a solid color. They just won't have the same wear. You'd have to "distress" the new wood to make it look "right". (Using old wood that's in better condition would be the preferred option, in my opinion.)
Do you have access to a local lumber mill (that's open) that could supply you with lumber of the required dimensions?
Replacing entire boards (especially the two on the right-hand side of the pic) will involve removing the roller hardware and the handle. Can you get those bolts lose or are they rusted into one solid mass?

Do you want to paint it and hide all the patina or do you want to put a preservative/sealer on it just to keep it from falling apart more?


You will want need to scrape or gently (very gently at first - hard to tell what kind of condition those boards are in) power wash the wood to get all the loose dirt and gunk off of it before attempting to apply an sort of coating. If you don't, a preservative won't soak in very well, and a paint coat will just fall off in the next stiff breeze. i.e. you'll keep busy for now, but you'll be busy again next spring fixing up the mess you left behind.
Personally, I'd go with just a preservative or sealer to allow the patina to show through. Even this will change the color of the wood some, but it will have minimal effect in comparison to a stain or paint. 
If you've got other painted wood on the barn and you paint this to match, that would make sense. Do you have, or can you get paint to match?

Do you want to spiff up all the hardware or leave it rusty?


I'd seriously consider hitting the metal work with a wire brush (drill powered, unless you're looking for a serious arm workout) to knock the heavy rust off, then paint it with a rust converter that will turn the oxidation into a protective coat. These are usually available in a variety of colors to coordinate with whatever color you choose to paint - I'd think just a simple black would look nice.
Converting the rust to a protective coat will definitely lengthen the life of the ironwork.
I'd include spiffing up the rail that it hangs from, too.
You could paint the iron work after applying the rust converter. Depends on taste and desired aesthetic, of course.