A 21-year-old cat is equivalent to a 100-year-old human.
The following is a chart comparing the ages of cats and humans:
1.5 months = 4 years old; 3 months = 6 years old; 6 months = 10 years old; 9 months = 13 years old; 1 year = 15 years old.
2 years = 24 years old; 3 years = 28 years old; 4 years = 32 years old; 5 years = 36 years old; 6 years = 40 years old.
7 years = 44 years old; 8 years = 48 years old; 9 years = 52 years old; 10 years = 56 years old; 11 years = 60 years old.
12 years = 64 years old; 13 years = 68 years old; 14 years = 72 years old; 15 years = 76 years old; 16 years = 80 years old.
17 years = 84 years old; 18 years = 88 years old; 19 years = 92 years old; 20 years = 96 years old; 21 years = 100 years old. 
How to care for a high-necked cat
Provide high-quality, high-protein food specifically formulated for senior cats. Senior cats have a much higher protein requirement than younger cats and adult cats. In addition to inevitable aging, illness and stress can also cause protein loss, thus reducing muscle tissue. Protein supplementation can effectively compensate for these losses.
