I am 76 years old. I have a California driver's license expiring on September 8, 2018 and presently live in Europe. I cannot be in CA on the renewal date. I probably will be in the US in spring 2019. Can I do it at that point?
If this is a California license, according to this private website you can renew it up to six months before it expires or 90 days after it expires (so December 7th 2018). After that you would have to apply for a new license. According to my searches, the DMV agent may choose to waive the requirement for a driving test - but they don't have to (and may have a prejudice against allowing a more mature person on the road).
Note that if you are living in Europe, you probably can't drive with the US license after a certain number of months. (The period will depend on precisely where you are, but a year is typical.)
Since California driver's licenses can be renewed online, your temporary absence from the state shouldn't prevent you from renewing before it expires.
If you received the renewal notice by mail in California, you may have other options such as renewing by telephone following its instructions.