This morning, I opened the front door to my children's school, let my small children pass under my arm, then went in after them but held the door open for the next person behind me (there was a line of people climbing the steps up to the door). It was a busy time of morning with teachers/staff/children trying to get where they needed to be.
The young fellow behind me (easily a foot taller than me, and much heftier than me) didn't take the door 'off me' i.e. take over holding the door open, walk through and hold it open for the person behind him. Oh no, he let me hold it, waltzed around me, pushed over my youngest child - and not a word of thanks or apology out of him!
Teenagers generally get a bad rap, in my view. On the whole, I find them interesting, funny, clever, and still quite cute with their big feet and smart attitudes and pretending-to-be-adults. But this lad! Security guy gave him a sharp word, thankfully. It's okay for teenagers to be a bit self-absorbed and not see the world around them, but outright rudeness isn't on.
Manners need to be instilled from the very beginning, so they become automatic by the time children are pre-teens. They've got too much other stuff going on in their lives after that to learn, which is how we end up with rude adults.