A stable upbringing with a family that you can use as role models. If you don't have that, you're in for an uphill struggle.
If everyone around you is unemployed, or works in part time unskilled labour - if you're moving from home to home all of the time - if you have no bedroom - if your family argues, or you don't have proper food so your blood sugar is all over the place - these are tremendous things to overcome in isolation never mind all at the same time.
People talk about 'privilege' in upbringing because it's actually kind of rare to have a functional family.
This is what the class system really is. Some people can jump the gap after their formative years, but most people are going to be stuck in those patterns if they've spent 20 years in the trenches.
I grew up in a poor family; my father left when I was young. The one thing we had (aside from the absolute warrior-woman of my mother), provided by the government back in the day, was a council house. A low rent secure tenancy. So whilst everything else was a struggle, my mother was able to provide a relatively stable housing situation for us.
Without that I'm sure I would be a complete fuck-up now.