Age of Consent

When people say "X or Y is 18 he/she is an adult. He/she can make their own decisions i.e. do drugs, have sex with whom they want etc.)" when casually dismissing any questionable acts of this 18 year old, I ask them that if the age of consent was 17 or 16, would they act the same way and if, say, the act was sex, would the the older party still engage. 

I ask this question because it's what I've observed in Western society. All of a sudden what is supposedly forbidden is now on the table without any discernment if one should do X or Y action. 

It's utterly bizarre to see nothing wrong with an 18/19 year old having sex with a 30 year old, especially if that 18/19 year old is exceptionally naive and not wise, yet has issues with people getting married in their early twenties and having kids before 25. 

I suppose this is the modern day feminist and the fruits of the Sexual Revolution - dulling the critical thinking skills and making all sexual things amoral unless there's consent of between the "adults." I mean, with this logic, don't get pissed of if two adults decide to have six kids - to say that one's a baby maker and they're actively destroying the earth with their carbon footprint would so immensely amusing.