You're an "adult" who can make their own decisions. Okay. Don't get pissed off if people eat meat, eats the occasional donut or two, believes in a divine and chooses not to live in either NYC, LA, London or Paris.

The talking of "he is an adult who leads his own life and makes his own life decisions" is relatively vapid. The phrasing always struck me as a bit smug and self-important. Why? It's uses the age of majority as some sort of standard when in reality that line is, well, arbitrary to begin with. Usually the phrase is used when someone defends questionable actions when someone more conservative objects. 

A 10 year old has enough mental capacity to know exactly what they want, granted they aren't making the same decisions that an 18 or 20 year old would make, but nonetheless making age appropriate decisions that are usually not subjected to a parent's order or wishes. Some examples would be what PG-13 movie to watch or whether or not to join Peter and Tim at the park to play some football after homework is done. A 10 year old can make a decision on whether or not to eat a pop-tart for breakfast with a glass of orange juice or wheat cereal with a glass of milk. As a parent, you usually respect either decision and rarely do you subject them to eat both a pop-tart and cereal while drinking orange juice and milk.

One issue with Western civilization is how they raise children into adults. 18 isn't a major number. It does not denote anything besides "legally an adult and as an adult you'll be charged as an adult if you commit a crime." If a libertine uses the "they're an adult - respect their choice and move on", then they should also use that same phrase to defend people who do stuff that's opposite to what they think is right, moral and ethical.