Kay, I feel your pain. Mandy, no problem,Lacey, oh boy. She is scared when I clip her nails and growls at me most of the time. She's never bit at me but I'm waiting for the day.
As for Simba getting older, he's got a long time left to go, I'm sure. An aging dog has many wonderful, special moments, you will get to enjoy. Best to try not to dwell on Simba getting older and enjoy him.