DS had it when he was just over 2 yrs old. DD who was 8 months or so a the time somehow escaped it. It was hands down the most terrible, horrific illness experience I've had with the kids in 8 years. If your child gets it, you *will* know it, if only by the smell of what comes out. DS was always failure to thrive, just couldn't gain weight. He'd finally put on a little weight over the previous 6 months, and in the span of 9 days lost 9 lbs. At 2 yrs old. We were darn lucky it didn't turn into something a lot more serious with that one!
From what I've read, most people get it throughout their lives. It's common for children under 5, but it's also not nearly as severe after age 3 if I recall correctly. For most adults it's just uncomfortable for a few days - though of course there are always exceptions. For a normal, healthy child I wouldn't even consider the vaccine personally. Now, I did hear that they're considering a campaign in Africa to vaccinate the children there against it, and that the numbers are very, very high in regards to infection and death. I really wouldn't be surprised if in some African countries the numbers are 95%! However, here in the US I know too many people who's children haven't had it to where either the bulk of that 5% is in my circle of knowledge or the numbers aren't really that high.