I recently read that, based on capitalization, US stocks no longer make up the majority and that to be diversified would mean to hold 50% or more in non-US stocks. Is this good advice?
Rephrasing your question: Am I diversified if I have more than 50% US stocks?
I would say that you can certainly be diversified and have more than 50% of your portfolio invested in US Stocks. I view the amount of international stocks (non-US) as a risk choice. My observations have been that my international stocks have higher risk which comes with a higher reward. I'm not comfortable with putting too much of my portfolio into a very high risk category.
I personally invest 25% directly in mutual funds that invest in foreign stocks. When you couple that with the money I invest in US stocks via mutual funds that have foreign interests (Coke, GE, etc.), I'm somewhat over 25% international in my portfolio.
Without knowing anything else about you, I'd say I need more information.
If all of your investments are in stocks, then that's not really diversified, regardless of how many stocks you own. There are other things to invest in besides stocks (and bonds, for that matter).
What countries? "International" is pretty broad, and some countries are better bets than others at the moment.
If you're old, I'd say very little of your money should be in stocks anyway.
I'd also seek financial advice that is tailored to your goals, sophistication, etc.