It happens. Anal glands often become impacted if normal bowel movements aren't expressing them regularly. Older kitties tend to have more bowel trouble (kind of like older humans), so anal gland problems can happen to the older ones too.
Yeah - glad it was something minor. My Loki goes to have his anal gland expressed every 4 months so it's not just something for the seniors. He might have to have it done on a regular basis - check with your vet and see what he suggests.