So, the idea is government bonds as an asset class are like very slow and steady gains, whilst providing you with income.
You take this slow and steady growth, as opposed to a stock’s super duper gain, so that when the stock markets go tits up, stocks crash and bonds will go up (or at least level out). And the reason you want that as you get older, is because you won’t generally have time to recover the losses from the crash in stocks, like the YouTube video said.
That’s generally why they’d advise for ‘mature’ people to get into bonds. And that’s generally government debt (US Treasuries, UK Gilts) because corporate bonds tend to act like stocks, which is a no-no.
That doesn’t mean that granddad shouldn’t invest in equities though, I don’t think many people go 100% into bonds, it just means that the vast majority of his portfolio should be into bonds depending on his age.