Around the equivalent of 60-70 in human years. Their medical science isn’t nearly as developed as ours is.
There isn’t too much difference between the colours, but there are a few oddities like purples (who have a disproportionately high suicide rate compared to other colours) and the wintergreens (who are not even close to being as technologically advanced as the other races, and live in a very high risk place).
There are plenty! Merry doesn’t encounter them though, being as isolated as she is.
Obviously that changes later in the story.
Her diet consists almost entirely of various mushrooms that she grows on the underside of her home island, occasionally supplemented by fhloots (the little green birds) that she manages to take down with her sling. She tries to conserve ammo as much as possible as if it flies over the side its gone forever, so they would probably be her favourite as a rare treat.
Also because she really hates them and their noise.