EDIT: don't listen to people talking about compound interest. If you have a 401(k) you will be paying this interest to the government bonds your 401(k) holds when we are finally in NIRP. Look at Greece today, they are going to have to pay $71M on July 17 which is just the interest on their 2014 3-year bonds.
If you are young, learn from the bad advice coming from the old folks.
EDIT 2: owning US stock, no matter when purchased, is a bad idea now in these times of share buyback, yes. So yes, it cannot be a good idea to hold stock now since we're due a crash (which got caught yesterday by a "system update glitch" at NYSE which curiously affected United Airlines and Wall Street Journal as well). Holding some gold and cash, as Bank of America advises NOW, is a very good idea: http://www.bloomberg.com/news/articles/2015-05-18/bank-of-am...