How much of anime and manga is actually intended to be consumed by adults?
Who cares? Do you need to feel that your hobby is appropriate for your age group?
Are you including otaku as adults if they are technically in the right age range? Because then it becomes surprisingly more.
Because obsession with purity and everyone telling each other that "It's nonalcoholic beer" gets really old. Same with the same old middle school and high school settings.
Most anime are for young children and those in the middle school, junior high, and a bit of the high school range.
Of course, there are exceptions and its not as if there is any unspoken rule that a 20 year old can't watch something made for 13 year olds.
Just about everyone reads manga, regardless of age, though.
Calm down. It's a legitimate question.