Answer this question however you'd like. Using the parameters and assumptions you like. This is more of a thought experiment. The gist of this is, if medicines allow us to live to be 300 to 500 years old. What are the chances we'd even get there without dying in a car accident or some other tragedy.
That's really hard to answer. I'd say that the total average might be not much different from now.
Humans tend to compensate security with more dangerous activities. So if you look at old WW2 veterans, they appreciate life much more than the younger generations. Young people often become bored from their lifes and start to do dangerous sports (driving motorcycles, downhill mountain biking, snowboarding and skiing, skydiving, hunting, eating jung food, drinking alkohol and taking drugs, collecting lots of STDs, buying lots of useless stuff until they are in a debt squeeze etc.)
So when the people don't get old, they also don't become afraid of doing dangerous stuff and they are much more likely to be killed in an accident. I wouldn't even be surprised, if people started to compete in voluntary hunger games, similar to those in movies.
Some people might become a few hundred years old, but many would die in an accident before they reach 70 years. The average age expectancy might be around 100 years.