Just depends if you're getting a lot of bad sectors or other errors. One of my hard drives I've had for 10 years, and it's been on for a total of 4.5 years with 2TB of writes, and it doesn't have any problems. But it's a lottery. You also used to be able to get consumer hard drives with 5 year warranties, I think most now only come with 3 or 1.
When people comes to my repair office, I always suggest to replace their HDD with a SSD. Then, add some RAM if they do more than Internet and mails. Each time, they cringe at the price and all, but once done, they always tell me that they will never go back to HDDs. HDD are nice when you need to store a lot of files, though.
Depends on how you use them, their condition and other factors that may not be tangible for the user. My external storage disks are about 10 years old, and they still work fine. I even have 22 year old HDDs that still work, but I guess it is because they have not been used much since the 2000s. With perhaps 16 hours a day use, I have seen two similar Western Digital disks fail after about 3 years of use. My disk from 2005 still functioned until the computer was retired in 2010, and then it was installed in my PlayStation 2 console. But because you have not used a SSD, I don't see how you can really tell that your PC is booting too slow for its standards... unless it really takes too long. If you're making that decision based on the rest of its hardware, the HDD is likely the bottleneck. My own PC is at least two generations older than yours, but Windows can start up in less than 10 seconds (in fact, even before the Windows 7 logo can finish its animation). It's never going to play modern games well though, since everything is still mid-range from 2013'ish.
Well, I did some test and yet everything came up 'good'. However I'm hearing my hard disk spinning up weirdly now. Happens a few times while it's idle, however it's not a clicking sound. I've ordered a SSD anyhow. I'll report back how things change.
It can be worth using something like: https://crystalmark.info/software/CrystalDiskInfo/index-e.html To take a look to see if the drive is failing. However modern drives seem to be terrible in terms of reliability, google published some research ten years ago http://research.google.com/archive/disk_failures.pdf
I used Crystal Mark and came back fine. My SSD arrived today, and performance has dramatically improved. So far, everything is great.