This was the general rule in my area. If you were a young man who (1) avoided getting into serious trouble with the elders (2) was regular at all meetings and (3) put in at least 10 hours a month in field service, you'd make ministerial servant by 20/21.
Maybe I was just always in the wrong croud, but I didn't know many young pioneers and everyone I grew up with that didn't get in trouble where MSes in their mid-late 20s. There's one uber-dub in the group, though, that was made an MS at 18, elder at 22 or 23. Man did that do a number on his ego. Its rare that I'll encounter anyone with a way of speaking that equals his in pretension.
I was publically reproved for drinking when I was 20 or 21. I can't remember the exact year anymore. In any event, I went from feeling like some giant stud to feeling like an outcast. My reputation had been destroyed. I began dating a girl a few years after my reproval and it was going great until her family found out I'd been reproved. You'd have thought her dauther was dating a drug-dealing, gang-banger with a felony conviction. It was awful.
A little off-topic, but it always struck me as shameful the way people would treat someone who'd been reproved, even if it had been years prior. Years ago I knew a girl that started dating some former bethelite that had been reproved about a year prior (of course, no one knew what for, but EVERYONE assumed it was fornication). A number of people questioned her choice because he'd been reproved. The contrast between this behavior and Jesus' words to forgive your brother 77 times (as well as many other oft-cited principles on forgiveness) was quite stark.