It's not a little post -- you asked an excellent question and raised a very intelligent issue that I think eludes most people. While may seem to you that "Rent" is a show only for teenagers and 20-somethings, gays, bi-s, etc... (and sure, that is perhaps the core audience), the fact is that there is no possible way that it would STILL be running nearly 3500 performances later (one of the top ten runs in the history of Broadway) if the under-30 crowd was the only one that responded to it. A significant portion of the straight, white, heterosexual, "older" traditional audience must have responded to this show or else it would have closed long ago. They probably haven't seen it dozens of times, but they're the ones that pay to see it for full-price and enough of them do so to keep the show running year after year.
The so-called real youth-oriented shows typically burn through their potential audience in a year or two (Hedwig, Rocky Horror, Tommy). ANY show that can run 8 and a half years on Broadway (with no clear signs closing soon) clearly has an appeal far beyond the "youth-market" (which cannot sustain a long run on its own) and must be attracting very large numbers of 40-somethings, 50-somethings and, yes even a few 60-somethings, the folks from Jersey and Long Island as well as the tourists from around the country and around the world -- I'm sorry there just aren't enough teenage Rentheads to explain it otherwise........ Larson wrote a score that screamed "ROCK" to his core audience, but wasn't so "ROCK" that it turned off everyone else (as, I think "Hedwig" did) and was accessible enough to the typical average theatregoer, which is why it's going end up running (despite the gay subject matter, etc..) longer than almost all of the safest and tamest Broadway hits of the last several decades.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney