I mean the person is the same person just he is 1 yr older, but the person could still behave the same way as when he was 18.
I dont see a big difference 1 yr. when it comes to the person attitude, behavior, maturity, etc One yr is just a number and that does not make a person do whatever they want just because of a number.
I mean for certain aspects of life of course being of legal age allow you to apply for things or do things that underage could not do, aspects like legal life stuff such as marriage (even if the person is still immature¡ for that) or other hundreds of things legally related that require the person to be an adult or applying for bank accounts,, smoke,, etc etc
But if the person continues to act like a kid or stupid, what does it matter if the person is 18, 19, 25 or 30? The behavior still is wrong if they continue to behave the wrong way.
One example (there can be many different exampels on different fields): A 19yrd old boy sleeping with his 18yrd old girlfriend at a family trip where other family members (including his sister and his dad) are part of this family trip. The couple are just boyfriend/girlfriend, nothing formal or serious between them, they are not living together either, they both live with their respective parents and they set a day to see each other again for dating. But they wanted to share the same room and sleep in the same bed like a serious couple, (they just been dating 1 yr anyway) while other cousins who were of the same age, never brought their respective boyfriend/girlfriend cause it was a family trip to spend time with cousins, family and other relatives and their parents also did not allow them to do, cause it was a family trip.