Learn English – “Year olds” or “year-olds”

"From one year to another" is definitely the correct choice among those three.

I would be more inclined to say it "from one year to the next", though.

You could also say "the price change from year to year" or "the year-over-year price change" or even "the annual price differential."

(edit: I should also say that while "from a year to another" may well be grammatically correct, it is absolutely not idiomatic and will sound bad to most if not all native speakers.)

If you are using it as a compound adjective or noun, as in your example sentence, it should be "40-50-year-olds".

If you are using it as a separate qualifier, as in BillFranke's suggested alternate wording, than it would be "those 40-50 years old".

Confusing, perhaps, but the general rule is that when any sort of counted "thing" is used as an adjective, the object of the count is singular. "40-year-old man", "3-mile run", etc. Making it a range instead of a single number doesn't change that.

But when a number and an object of that number are used "on their own", i.e. not as a compound word, the normal rules of pluralization apply: "those 1 year old", "those 2 years old", etc.