There have been a lot of rules of thumb over the years. 50/50 and 60/40 used to be common advice, no matter the age of the investor. When modern portfolio theory became influential, the "age in bonds" rule of thumb became common. Now some people, e.g. Burton Malkiel and others who are involved with the robo-investing companies, say that young people don't need bonds at all, and that high-dividend blue chip stocks can be a good substitute: https://blog.wealthfront.com/investor-bond-market/

There's no simple answer. There might be a right answer, but so far no consensus view has lasted for long. Most of the arguments I've seen have been heavily dependent on bond yields at the time the arguments were made, and are therefore not universal advice. For example, 50/50 made a lot more sense when treasury bonds were paying more than 7% per year.

I take no position on this particular rule of thumb but to note it is much less aggressive than more broadly held advice on the subject.