Stocks and shares are risky even if you are 18, 28, 38, 48 or 58!
The issue is not so much at what age do you invest, but rather, what are you using shares for? Speculation or investing for dividends and growth?
The reality is that the thin line separating speculation and investment tends to be blurred for both amateur investors and experts alike.
I think if you are mature, guided by more experienced people, to invest in shares at 18 is okay only if it is money you can afford to lose. To invest your parents' monies or borrowed monies is definitely a no-no.
The more important factor is no matter what age you are in, are you prepared to lose everything when you invest? The mindset is important!
Be well and prosper.