One day I was holding a tutorial with a group of my students at the uni and I "forgot" the exact word I wanted. I could remember similar words and I could also "talk around it" so it became a bit of a joke in the group to guess the correct word. I said "well Im in my late 40s so it must be old age creeping up."
One student said "I hope not. That happens to me and Im only 24!"
There are quite a few quizzes on the internet you can do to determine whether it is likely to be Altzheimers or similar. If you forget a word or name and can remember it later. Or can explain what you mean its just the result of natural "forgetting" that comes with age.
If I had to write my Ph.D now I would find it harder as I would have to depend much more on my notes. Whereas before I could hold complex arguments in my head. However I have no doubt that I could still do it.
