The current Mac operating system, High Sierra 10.13, is offered for free via the Mac App Store and everyone is prompted regularly to update.
But should you take the offer?
High Sierra is designed primarily for computers with SSD storage. That is a Sold State Drive, as fitted to all new laptops and on iMacs if specified at purchase.
iMacs also may have Fusion Drives which have a part of their storage on SSD and the rest on an old fashioned regular hard drive.
High Sierra has a totally new type of file format which is unsuited to the older, slower hard drives. Installing High Sierra on older Macs will slow everything down unless you have an unusually high amount of RAM, at least 16GB. This enables the computer to cope with the new system.
New Macs have 8GB as standard and older ones often only have 4GB.
On a 4 or 5 year old Mac with 4GB of RAM you will find that High Sierra runs like the proverbial canine and an old mutt at that.
So as a rule of thumb, if you don’t have a SSD in your Mac don’t go past Sierra 10.12 and your beast will be a lot happier.