Some people have been commenting about the huge differences between time of AIDS-onset in different HIV-infected individuals.
What makes your immune system better at controlling HIV is your MHC capacity for presenting the antigens. The viral antigens have mutated to i) avoid triggering an immune response so it won't be well degraded in the first place, that happens by avoiding inflammatory pathogen associated molecular patterns (gp120 bings to a crappy receptor called DC-SIGN); ii) not fit well in most people's MHC cleft's, meaning that even when the pathogen is properly degraded, if you don't have good affinity with the MHC cleft, you won't be able to mount a good response based on T lymphocytes.
The item II is what usually impacts the viral fitness in clinical practice. People with the same promiscuous MHC clefts which can cause autoimmune diseases are the ones who are usually slow developers. Since they can mount a good cellular response from the start, they will easilly get rid of most copies of the virus. What will be left will usually be the ones who have had mutations so big that they will not only not fit in the MHC cleft but they also won't bind well with CD4 or the correceptors. I've taken an extreme case, but this will happen in people with standard MHC`s who mount a standard answer.
Also, the interesting thing about elite developers is that they don't mount the standard cytotoxic response which is expected against viruses. The dominant answer will be based on a novel type of T CD8+ nicknamed non-cytotoxic (duh) lymphocyte. These will, instead of killing the infected CD4+ lymphocytes, secrete anti-viral factors which will stall the virus reproduction. Another interesting example which shows that many diseases which we have are less harmful than our immune response. In this case, killing the infected cells means killing the own cells which will keep your immune system working, so it's a terrible idea, since the virus evolves faster, changing the antigens very fast while our immune system takes too long to mount a specific response. you will eventually suffer from immune exhaustion and that's AIDS.