Bitcoin is the best form of investing for retirement. I don't see any other retirement plan better than Bitcoin in the world we live in. Anyone who has not invested in Bitcoin for retirement is not setting a standard for when they get old. Many will be glad that they invested in Bitcoin 50 years from now, and many will rejoice because they are living comfortably with their retirement plan.
Other assets for investment for retirement are real estate, building a plaza, and owning an engineering company that deals in heavy machines. Gold and silver are also very good investments for retirement.
Generally, a good practice would be to make retirement plans for atleast for different kinds of assets/stocks, that way you don't end up losing everything if one asset or stock goes bad. For bitcoin investment, I practice a modified form of naira cost average; I buy aggressively when there's a bear market, then buy less aggressively when there is a bull run. So at the time when I am buying less aggressively, I channel the remainder of my investment funds to other asset.
The stock market is dangerous for someone on a retirement plan. Instead of investing in the stock market, let them invest in government bonds and mutual funds. Mutual funds are safer because the investment returns profit monthly while the capital remains intact, till the owner demands it. Mutual funds' annual return can range from 10% to 20% and even more. Divide it by 12 months. The amount you get is what you will be paid monthly.