In today's "Ask a Fool" series, Daniel asks one of our senior analysts how much of a young professional's income should be invested in stocks. 

After checking in with our personal finance guru Dan Caplinger, Eric suggests being more aggressive than the traditional 5% to 8% that many advisors throw around, and thinks young earners should aim for 10% to 15%, if possible. 

Of course, every investor also has to take his or her individual circumstances into consideration when making these decisions. If you're saving for a house, car, or other large purchase, your percentages could be different. 

If you're at a loss for which stocks to select, but know you'd like to be invested in the market, you can always look at some broad market ETFs, which will passively invest in broad indices like the Dow Jones Industrial Average (INDEX: ^DJI) or the S&P 500 (INDEX: ^GSPC).

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.