With a few exceptions, it is best to allocate a significant percentage of net assets in carefully selected common stocks or stock mutual funds. The old rule was to simply subtract one's age from 100 to get the percentage one ought to invest in stocks. However, as national debt has increased (making for greater inflationary pressures ahead), health care costs have risen markedly and likely will continue to do so, and people on average are living longer, that formula needs to be adjusted in the direction of a larger stock holdings. So it is now suggested that the appropriate formula is 110 less one's age as the correct equity percentage.
This can be modified, though, by related considerations. If one is quite risk averse, a lower stock allocation may be necessary. On the other hand, if married to someone significantly older or younger, the percentage chosen for you as a couple would be the average of the formula results for each of you, which would then be higher than indicated just for the older partner. In addition, if one is receiving a pension, a retirement annuity, ongoing disability payments, or Social Security checks, this may be treated as income from the non-equities segment of the portfolio, so that the stocks portion of liquid assets can compensate by being greater.
Finally, if one's investing skill level is low or high, that can be factored into how little or much one holds in cautiously chosen equities. These days, the returns from money market funds or Certificates of Deposit (CDs) are low enough, and the risks of owning bonds or bond mutual funds once interest rates begin to head back up are substantial enough, that it pays to know enough about stocks to be able to have a bigger part of the financial pie in equities, especially since in general they tend to keep up with inflation far better than reserves or bond assets.
A useful strategy is to invest in reasonably safe type stocks when periodically they are low in price per share relative to their worth and then, after awhile, replace them with others that then are more attractively priced. Often these stocks also provide healthy and growing dividends, thus augmenting one's income supply. (See table for 5 that I think are good purchases at this time.)
One can also profitably buy shares in the very best companies, when available at a discount to their true value, and merely hold them long-term or even "forever." In this category might be companies such as Berkshire Hathaway, General Electric, MacDonald's, Harley-Davidson, Coco-Cola, and Hershey's.