I find it extremely common to meet people in their 40/50's who are still will their partners 10/20 years later even longer.
I just feel that younger people seem incapable of holding down a job let alone a relationship. Most of my friends are on their 2nd or even 3rd serious relationship & I'm only 26!
I met my bf at 16, split up 3 months later. I then met someone else at 17 it ended 6 months later. Then another at nearly 18, it lasted 2 months.
Me & my first bf when I was 16, got back together when I was 18 & split at 20. 21 I met my ex wife, spent 5 years together & split in 2015, divorced in 2016.
Met a new guy in 2016 & split 7 months later.
Are most of my generation incapable or just unwilling?? Maybe we aren't made of the same stuff!
What on earth makes you assume that any of those people who have been in a relationship for 10-20 years in their 40s and 50s didn't have the exact relationship history you're describing here? 
I think my relationship history would be something like:
One month at 16, two months at 17, around a year at 20, fuck buddy arrangement with a nice chap for a few weeks in the rebound at 21, ill-considered trial of a relationship with someone else for about two months a bit later that was never going to amount to anything, three-five months long distance at 22, random unusual sexual encounter for the sake of pushing the boundaries a few months after that, then got into a LDR with now DH just after that, and we've been together for five and a half years and quite possibly will become those people in their 40s and 50s you speak of. DH's relationship history is far less eventful, though, admittedly...