I’ve been made fun of for years because I feed some cooked chicken, fish scraps etc to my cats, and choose their canned and dry food carefully, reading the labels-but 18-20 is a normal, healthy life for them-some have even been older-I must be doing something right...
Far as we know our girls are around 13 and still going strong. One sometimes acts like she might be getting a touch hard of hearing, but inconsistently enough we’re not sure...other than that, they both act like kittens. I wish I could say the diet was something special but it’s whatever kibble is on sale any given week because that’s what we can afford.
I don’t doubt your cats enjoy it, and it’s most likely very good for them as well, but I’ve seen cats who had the best of care decline at 10, 11 and the cat versions of hardcore skid row cases who made it to their upper teens or even 20+.
I suspect genes have more to do with it than we like to think...