Can a successful vasectomy ever truly fail at all?

Hi all! I had a vasectomy back in November of 2004. I had a semen test at 4 months, at 6 months, at 8 months, and then at a year and at year 2. They all came back sperm free by the urologist. Can a successful vasectomy ever truly fail at all? In my case, do you think I'll be totally fine even when I'm over 100 years old. Any answers would be appreciated. Thanks!