For many purposes it would be cost prohibitive to run detailed background checks on individuals to discover every relevant fact like that, so they rely on aggregates, which works well enough.

I'm sure there are, but it's kind of standard that most 18-21 year olds have only 1-2 years experience in an independent apartment/house. Just like 17 year olds typically only have 1 year experience driving.

I wouldn't even rule it out entirely, you'd just need references (a decent rating).

The same age group is faced with similar problems renting apartments because they often lack credit ratings and previous references.

Maybe the car insurance example was bad, but I think the situation is a little more complex than pure statistics.

The US has a very long history of racism against minorities. It's been difficult to get a strong system in place to combat racism. Ageism against young people has a built in correction system called aging.

But is that not what the review system is for? What prevents a host from reviewing potential guests' history on a case-by-case basis to determine their level of risk?

Black men are more risky because of a history of systemic racism forcing them into situations where they need survival and status systems outside of the law. Young people are more risky because young people are impulsive and stupid. So it seems appropriate to treat these two things differently.