You see there is this thing called the circle of life and it moves us all. To everything turn, turn, turn. Ok you can smack me now. Yes, all living things, including trees, have a lifespan and then they die. I think orange trees live 30+ years though.

Our citrus trees (Navel, Satsuma, Duncan grapefruit and some kind of lemon) are all 30 years old. They were planted in the fall of 1980 when my folks moved in. :)

It sounds to me like something's eating it. Have you tried the Bayer Tree and Shrub treatment soak? It works fantastic for the problem you are describing - if it's insect based. It treats for bugs, but also fertilzes it.

If it's a virus (which it doesn't sound like...), then you will know because the Bayer product won't do anything.

My citrus is pooping out now and were planted in the 1930s.

If all else fails, take some good pics and email them to the county extension office. I did that and got an immediate response.

Our navel orange has thinning leaves and had much smaller fruit this year (just as many, but much smaller). Two reasons, I think:

1) January 2010 was very, very cold with several long, hard freezes (Jacksonville)

2) September-November 2010, we went 42 days without rain

These are not expert-based; just my opinion.