Thank you very much for your explanation.No, the difference is that 10 years older is stating a comparison. The usage 10-year-old to describe age is something that has evolved as the normal way of saying 10-years-old (which if you think about it should be the correct way of saying something is 10 years old). Describing a car as a 10-years-older car is not everyday English. The normal way of saying it is "his car is 10 years older than ...". I'd go so far as to suggest that it is a very odd way of saying that one car is older than another. Indeed, I can find no instances of either years-older or year-older in the British National Corpus, and without the hyphen there are no year older or years older followed by a noun.
This is a transcription of a letter written by Fowler (of Modern English Usage fame) to his brother. He didn't hyphenate twelve year, so what is one to think? Of course, we do not know if the transcription was accurate.than his twelve year-older partner