Yes, it's a bad idea to feed a senior (or adult cat) kitten food.  In fact, ANY
food that says "all life stages" in the AAFCO statement means that the food has
been formulated for kittens.  Kittens have a higher requirement for certain
nutrients.  Especially nutrients like phosphorus, sodium, etc.  Older cats
don't need the same levels that kittens need and in facts feeding an older cat
these higher levels can put the cat at risk for various disease
conditions--heart disease, kidney disease, etc.