[quote=@Dervish]
I always interpreted it as altmer can live for 1,000 years because they're the most "pure" elves, while the others (including orcs) can live for around 200-300, depending on life conditions. I don't imagine many orcs die of old age because of how brutal their lives typically are.
[/quote]

It's never really touched upon how culture effects the lifespans of races, for instance, a Cyrodiilic Orc lives a typically peaceful life compared to those in other cultures, as well as has access to Imperial standards of living. I always interpreted the Altmer live so long because, yea, same line of thought as you, they are the closest related to the Aldmer of old. Orcs are elves, perhaps Altmer? Who were turned into Orsimer by Boethiah. After that is when Shor, IIRC, curses them by shortening their lifespans. I don't know if it ever says how much of their lifespans the curse took, though. I always just thought they lived as long as men, that seemed a suitable punishment for Mer.