What Do I Write About?
What do you know more about than anybody? What is it you wish you could say to the world? What do you care about? Do you love books? Do you take pride in your garden? Do the most ridiculous things happen to you on a daily basis? Do you read the paper and think you'd make a better columnist because...? What have your friends and family been begging you to write about?
That's what you write about. You don't have to write about your personal life in your family. (Such topics have been a source of trouble for writers for centuries.) Of course, you can if you want to. (But you should consider writing anonymously and discuss this with your loved ones.)
Find that one thing you know better than anyone else and start there. If you make a great cocktail, start a blog about bartending. If you have an unnatural ability to rescue dying plants, start a blog about gardening. (My dad should do this. Just saying.)
Think of your blog as a go-to place to share your knowledge with people who are in desperate need of your insight. Because, as an Old Person, you have plenty of it. It would be a shame for the clueless Young People to wander life without the valuable information you have gleamed over time. It's time you made the world a better place.