In general, homebrewers make their beer at a strength consistent with the style of beer their making. No, but your friends might if you make them drink it. If you are not careful with your cleaning and sanitation bacteria or wild yeasts can infect your beer and render it undrinkable. In general, organisms that can hurt humans cannot survive in beer. The types that do survive are those that produce off flavors or high carbonation.
Beer is better when it's fresh, but there are exceptions.
Lagers require a few weeks to a few months aging to mature properly. Very strong beers, like Dopplebocks or Barley Wines also improve with some aging. Beers age differently. Budweiser and Miller stay fresh longer than most all-malt beers due to the use of corn or rice sugar. A born on date tells you when it was bottled, but doesn't tell you if you can expect it to be fresh.