It's not so much age as use.
A HDD that has been sat near idle for 5 years may last another 5 years no problem. One that is a boot drive on a server that doesn't have enough RAM so is getting thrashed 24x7 may last less than 12 months.
As mechanical devices, they wear out
My oldest data centre was opened in 2000. There are servers that we put in on day one that have never seen a HDD failure. Likewise I have seen drives fail in the first few weeks.
But last year I spent some time with Google and they have so many drives and such control over tens of thousands of drives that they can tell you which drive is going to fail next to a level of accuracy that is incredible.
Often this is down to information based on batch etc.
That's my point - age is one factor.
Environmental factors also have a bearing as well as use.
For example, drives in a tougher environment seem to be more likely to fail. Drives which get too hot and drives which are subject to shock seem to be more likely to go.
I happily replace hard drives in PCs with ones from other computers which have been scrapped or died (for other reasons than hard drive failure of course - any whiff a hard disk is dodgy and it's "oot"), and they can live on without issue. One shouldn't suddenly be more concerned as a result.
The bottom line is, it is a lottery to some extent and provision for backing up should be made no matter what storage medium is used, as they are ALL subject to failure (or disaster / theft etc.).