Yeah we shrink as we age, I used to be 5'8" but due to back injury and arthritis I am now 5'5" at age 60. Bikes come and go, its not like its a dog. You can sell them and buy new and love the new one as much as the old. I have had 3 in the short time I have been back riding bikes. I was very glad to replace the Huffy with the Specialized, and the Specialized with the Day 6 when my body failing made a more upright seat important. The important thing is to keep on riding.
But the thought you would still be riding the same bike is probably nil. They wear out if you ride them enough to worry about it. Plus barring injury the regular bike riding should help to keep the shrinking down. Of all the things to worry about this is not one I would choose. LOL