Q: I have an early 2008 iMac (OS X 10.9.5), so I can't upgrade to Sierra. Is it too late for me to upgrade to Yosemite, or am I S.O.L. with an out-of-date computer?

You can go to El Capitan if your system meets the minimum requirements

<https://support.apple.com/en-us/HT206886>

You can upgrade to El Capitan using the link Bob posted. I am using an early 2008 iMac with El Capitan and it runs smoothly. I have upgraded to 6 GB of RAM, which I would strongly suggest you do.

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.      

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum.

Crucial

Other World Computing