Gas Water Heater Maintenance

My GE Gas Water Heater is now 10 years old and I'm wondering how long it will last. I have never flushed/drained it or checked the rod. I know... please no lectures! I didn't know I was supposed to do maintenance and I've never had any problems. Currently, no drips, water is plenty hot. So my questions are: 1. Is it too late to do any maintenance (i.e. drain, check rod, etc.) or can it do more harm than good? 2. How can I determine how much time it can last beyond the standard 10 years? Any plumber is gonna tell me to replace it cause they want to sell me one. Thanks for any replies!