Having done more searching over the last month, I thought it might be helpful to others if I answered my own question. I came to three realizations:
First, this is like asking, "Dear Google, when am I going to die?" It's impossible to know for sure and it's unnerving to even ask.
Second, searching for "older people with cerebral palsy" on the internet is a tricky proposition. Suppose that a person is 80 today (in 2016), and living with cerebral palsy. That means he/she was born in 1936. Would a person with a mild CP impairment even have been diagnosed as such, given the social stigma? Unlikely. Would such a person be tech savvy enough to be posting on message boards? Also unlikely. Would such a person be posting specifically about CP? Extremely unlikely. Looking for such people is like hunting for unicorns.
All that said, this comment thread features several older people with CP chiming in. I also found a few scattered forum posts from people in their 60s and 70s living with CP. The oldest person I was able to find was Maureen Arcand, who wrote extensively about living with CP and lived to be 86, despite considerable mobility impairments. That's actually above the current US average life expectancy of 81.6 years for women.
Third, there has indeed been some research on this topic, though it can be hard to find. Hutton and Pharoah (2006) distinguish between cases with and without severe impairments, and found that in cases where there were no severe impairments (i.e., able to stand and walk, able to care for oneself, no comorbid disorders such as epilepsy), survival outcomes were similar to the general population. Strauss et al (2008) found that in their highest-functioning category, survival rates were slightly reduced compared to the general population, but they go on to caution that, "if a person's pattern of disabilities is at an extreme end of the range for a given group, their life expectancy may differ substantially
from the group average." As Hutton says in this summary document, "A child who is mildly affected by CP can expect to have much the same length of life as
a child without CP."
To sum up, for those of us fortunate enough to be on the extreme mild end of the CP spectrum, all available evidence suggests that the CP itself doesn't put a drag on our lifespans. If you go to Dr. Google, you can find CP resource websites claiming that CP "puts extra strain on the heart" or that people with CP "don't build up a reserve of energy when resting". That may be true in more moderate or severe cases, but it seems to be an unlikely risk factor for mild cases.
Will I live to be 101, like my grandfather has done (and, unaccountably, continues to do)? Probably not, but then, that's true for most people. Just like everyone else, I don't know how long I've got, but the research says my odds are just about as good as anyone else's.