I was 19 and had 500 bucks, went down to my local discount brokerage, Discount Brokers (Internet wasn't around yet and Discount Brokers is no longer in business) and bought all the Staffmark stock I could (now trades under the ticker EDGW and much lower levels). The only way to really keep up with your position was in the newspaper on a day to day basis. I closed out the position a couple of months later and made 350 bucks. I'll never forget that, my first market investment/trade and profitable at that.
That was about 16 years ago and I've been trading/investing ever since but full time for the past two years and profitable consistently. I have always been in the markets in some form or fashion over that whole time span. Stocks, bonds, options, real estate and commodities.
Never too young to start.