Hi Imonroe, welcome to the forum.
58 + 15 years = 73 years. Life expectancy for a 58 year old is 25 years (from
https://www.ssa.gov/cgi-bin/longevity.cgi). So your life expectancy is to age 83. More or less depending on your current health and future lifestyle.
Your tumor is about the size of mine. Your symptoms sound a little worse. How bad is your hearing loss?
As you know, you have three choices: microsurgery (craniotomy), radiosurgery (radiation) or conservative management (watch and wait).
What I am doing is lots of research, regular hearing tests and some visits to neurosurgeons and radiosurgeons. The intention is to do this and not act until my 6 month MRI. Of course, a change of symptoms may change this approach.
If there is significant grown, I'll do my preferred intervention (currently gamma knife, but is still changing). If it has no growth, I'll keep observation for a couple more years. If it shrinks then all is good. The following probabilities are from
http://www.bhtinformatie.nl/pdf/ingrijpen.pdf . "significant growth in 20-45% after 3-5 years of observation", "Tumor regression or shrinkage is reported in 4-22%". So rough numbers are growth 30%, stable 60%, shrinkage 10%. You get a very wide range of probabilities depending on what you read. Also definition of growth can vary from absolute numbers of 1 mm to 2 mm to 3 mm and rates of 1 mm/year, 2 mm/year or 3 mm/year. If you are looking at numbers from a specific study, take into consideration the definition of growth.
My suggestion is to research as much as possible and talk to both surgeons and radiosurgeons. In your case, it looks like you can afford to wait until the 6 month MRI, but that will need to be confirmed by someone that is qualified. I recently re-read a contemporary paper (March 2016) that I find worthwhile (
http://appliedradiationoncology.com/articles/interdisciplinary-management-of-acoustic-neuromas). Full of information and references.