A recent issue of Kiplinger's magazine offered some eye-opening statistics about online trading. Check out a few:

  • Number of online brokerage accounts: 11 million

This is interesting to me, as it's a relatively small number. More than 40 million Americans bank online now, to some degree -- yet apparently, just about a quarter of them (probably less than that) trade online. This reflects the facts that (a) more Americans need to begin investing and (b) those already investing should consider the conveniences of online trading. (Trading online is easy.)

  • Average size of an online investor's portfolio: $189,000
  • Average age of an online trader: 49

These together are a bit worrisome. Having $189,000 worth of investments puts such folks well ahead of the average American, I'm sure. It's to be commended. But if you're 49 and you hope to retire in around 15 years, it's not enough. If that sum grows at 10% a year (and there's little guarantee of that and a good chance you may earn less), it will amount to around $789,000 in 15 years. That's good, but it's probably not enough on which to base a comfortable retirement. These folks will need to keep socking money away -- and should consider shifting some funds into short-term investments as they grow older. Learn more in How Much Will You Need to Retire? and Do You Want to Work Forever?

  • Number of trades made over the past 12 months by the average online investor: 30

This is on the high side. It suggests that either investors own too many different holdings, spreading themselves too thin, or they're just jumping in and out of their fewer holdings way too often. (Remember, BerkshireHathaway's (NYSE:BRK.A, BRK.B) CEO Warren Buffett has suggested that the best time to sell is "never.")

  • Average trades made over the past 12 months by "active" investors (who trade at least five times per month): 192

Yowza. This amounts to about 16 trades per month, or roughly one every trading day or two. And that's just the average number. While some surely trade less frequently, many trade even more frequently. If your trading commission is an ultra-low $5 per trade (a rate available via our Broker Center -- see what else it offers you), you'll be forking over a whopping $960 per year just in commissions. For those of us paying around $10 to $20 per trade, it would total nearly $2,000 to $4,000.

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Longtime Fool contributor Selena Maranjian owns shares of Berkshire Hathaway.