There could be many different answers. It could depend on what riding style you have, where you live, price range, etc, etc. But for all people, of all age, of all regions. Do not buy the piece of shit department store bikes. You know what I'm talking about. Bikes from Walmart, Target, Kmart, those places. Those bikes are complete and utter shit. The wheels are weak and bend (sometimes taco), the brakes will glaze, the grips will rot, the components will wear quickly, the seat is uncomfortable. And most of all, the thing weighs a metric shit ton. Bikes like these
Good Luck not buying crap! And a picture to make up for the one above.