There's a saying, past is not prologue. Of course it's true. But here's a history lesson, nonetheless, on the price of gold versus the Dow Jones Industrial Average (DJIA):
On January 21, 1980, the price of an ounce of gold was $850 while the DIJA was valued around the same time (January, 25, 1980) at 876.11. Today (37 years later), the price of gold is $1275 (up 50%) while the DIJA is 24,746 (up 31 times).
Think about that for a second. If you had been 30 in 1980 and had managed to squirrel away $10,000 which you then decided to invest in gold for the day you turned 67, your $10,000 would be $15,000 today. If you had instead invested in the DIJA, your $10,000 would be $310,000.
What's the point? Well, there are a couple. The first is that the admonishment, past is not prologue, bears repeating. There is no guarantee that the next 37 years will look like the last, in other words, That said, over the long term, stocks have outperformed every other asset class. So, while it's not an ironclad bet, it's at least one based on historically sound data/logic. As for your own portfolio, the amount you should invest in stocks depends on a few factors;
1) Your temperament (tolerance of volatility) first and foremost
2) How much you have relative to how much you want to have
3) How much time you have to accumulate what you want to end up with
It really is that simple so don't let anybody try to convince you otherwise.