That's not true at all. It doesn't really say in the books, but there is nothing that assumes most of the dwarves in the hobbit had any combat experience before the story. Maybe only one or two. Fflewddr certainly didn't have grand adventures before joining the story.Nearly all of these characters had adventures before they join the story. None of them are level 1 at the time the story begins.
Secondly, my overall point wasn't just that why aren't there older level 1 PCs, but that there are also no older high level PCs either. As I said, you can get to high level after only a year of adventuring. If most PCs start at 18 years old-ish, then they'd be level 20 at age 20ish. Hardly anyone plays an older PC (non wizard at any rate), and yet, there are a plethora of older (middle age+) of active heroes in literature and lore