Talking mostly about Pollenosis here, other allergies aside:
Shit didn't kick in till I was twenty-something. But when it came, it came hard. This year, it's almost been bearable. But the years before, I've been in constant misery for the better part of each year. It's not just Rhinits, or "stuffy nose", but also bronchoconstriction, irritated eyes and skin - for months at a time. You annoy and disgust others with your constant dripping nose and coughing while sleep deprivation will slowly turn you into a miserable piece of shit zombie.
When I was younger, I'll admit, I occasionally made fun of the kids that were plagued with allergies because it's been almost exclusively the kids that were raised in the city. Well, if that's karma, then it's really a bitch.
People who aren't plagued by this crap can't imagine how much this can affect you life's quality, I know first hand now.