Also, the early boarding is for whatever people interpret it being for. Most airlines are intentionally vague about justifications for early boarding. "If you need extra time or assistance to board...", and lots of people take advantage. I know a particularly "chutzpah" guy (who is young, has no disability, nor children) who always boards during this period because "nobody's stopped me!"

It also applies to small children, in my experience. But if you're flying with a toddler it's a lot easier to convince the gate agent to help you sort it out before boarding begins.