It is true that there are plenty of formulations of various foods and pharmaceuticals (also pesticides) that degrade over time, and even some that decompose into harmful as opposed to less effective or less pleasant substances. I cannot answer for all sunscreens being indefinitely useful and harmless, and they do differ in their active ingredients, some of which might well decompose somewhat on standing, but I would not dream of ditching a bottle because it was last year's purchase; last decade's purchase maybe, but by that time the emulsion probably would be going off too, never mind the active ingredients.

Many reputable companies make a practice of recalling all "expired" stock whether it is necessary or not, thereby protecting their reputations for quality. However, you have no reason to swell their coffers by pointlessly following their example.

Years ago when I had a boat I had a box of part used sunscreen. I certainly used stuff that was five years old, it worked and I had no ill effects. I once found some nasty looking pink stuff growing on the surface of a zinc oxide cream in a glass jar, I scraped off the surface and used the rest with no problem.

Having grown to be a little wiser over the years I might have some doubts about stuff in plastic tubes. Over time it is possible that some rather nasty compounds may migrate out of the plastic and into the contents. This may be why manufacturers remove old stock from the shelves.