My old Macbook 5,2 2.13 GHz Intel core 2 Duo says there is no software upgrade for it when I click on Software Update??????? I'm too old... but is my MacBook 5,2 old too for the new OS?
No, but you'd really, really want to max out the RAM before installing it.
I don't know why the Mac App Store isn't telling you that you could upgrade to Yosemite for free, but your Mac is eligible. Maybe you're using the old Software Upgrade which was separate from the MAS and thus wouldn't tell you.
As you've missed out on four major upgrades of OS X, there is simply not time in the day to explain to you all the things that you've missed. I don't know your definition of "world-shattering" but being able to answer calls on my Mac via relay from my iPhone elsewhere in the house is a pretty big deal that's made a nice difference around here. There are pages and pages of other features from across the various releases, but maybe reading about them is less impressive than when they are part of your daily routine and you wonder how you functioned without them.
Jumping from so far back is bound to cause some teething issues at first, necessary program upgrades and that sort of thing, but as far as I'm concerned its definitely worth it. My 2012 MBP can now run for the better part of a workday on the battery, particularly if I keep the screen dim (which is easy to do in the sort of dimly-lit watering holes I tend to hang out in!). To me, that's amazing -- and a huge productivity boost.