13 years isn't that bad. After all, lots of women live to 85 and their husbands die at 73. You could still outlive him. According to recent statistics the life expectancy for US men is 73.4 and women 80.1.
But also - ask yourself, if you had 15 great years together and then broke up, would that be ok? Nothing is guaranteed. He could get lung cancer like my boyfriend. Don't borrow trouble from the future. So long as you're in reasonably similar stages of life (i.e. he doesn't want more kids, you don't want to retire tomorrow) and you are enjoying each other, it should be no issue. It does sound to me like he thinks you're nt serious about him because he's been carrying that comment of yours around in his head all this time.
