I didn't have a key as a teenager, combination of parent or grandparent usually in, plus back door rarely locked, living in small town so if house empty key hidden under a stone (or failing that we could slide the downstairs dash window open and I could boost my sister up and she could wriggle in!!!
Parent usually picked me up from university, or if I came home on the train would pick me up from the station so no need to have a key while at uni. I did get lent one if I was going out later when I was older. I usually let them know when I was coming home, but did occasionally just turn up and ring the bell as a surprise, and they were always pleased to see me.
This changed when parents had a, walk in burglar not long after we got married. They started locking up even when they were in, so then gave me a key. I live 3+hours away, but still have a key, and would just knock but let myself in when arriving for a visit, and just treat the place as home while there.
DC have had keys since they started secondary school. One has boomeranged back home since uni, the other has stayed in their uni town, but both will always have a key to wherever we end up living. They might not always have a bedroom exactly as it used to be (got my eye on a craft room and DH wants a much bigger home office) but they will always be welcome and there will always be a guest bed of some description... Or if we do downsize to a 1 bed flat it might have to be an air bed or a hotel nearby, but I'd always want them to be welcome.
I'm so sorry for you OP, and others who were made so unwelcome. X