To be honest 17 is very young. I'd employ neither, without something that blew me away. I don't know that making that comparison is really relevant.

30 years and no work history implies some problem that has prevented them from working.

17 years implies a lack of life experience and likely a lack of soft skills. Once they hit their 20's however they're going to be less of a gamble, regardless of their work experience.

So yes at some point too much time will have passed for them to become more employable. But I was only thinking in a sub 5 year term. More as a means of encouraging someone in that situation, not as a hard and fast rule, that the older you are the more easily you're employed.

Hey, whatever you like! Who's more employable?

(1a) 17 years old; high school graduate; never had a job.

(1b) 22 years old; high school graduate; never had a job.

Or, if you prefer:

(2a) 17 years old; college graduate; never had a job.

(2b) 22 years old; college graduate; never had a job.

I submit that in every case the 17-year-old is more employable.

How would you go about finding a 17y/o college graduate in the first place?

I'd disagree, the 22 year old in every case to me.

You wouldn't wonder what he's been doing for the last 5 years?

Yeah I would wonder, but it wouldn't automatically raise concerns. I'd imagine that the unemployment rate for that demographics is relatively high. 22 is still very young and I'd not be surprised if many had not yet had a professional job at that age.

Perhaps my views are not typical, but I find a lot of maturing happens in late teens and early 20's. I might be jaded but I find people get significantly easier to work with when they hit mid 20's. There are many exceptions to this however and not all people mature at the same age.