Are there any diagnostic tools for adults for predicting future brain issues in older age?

Future brain issues is a broad category so there is no way to predict this-it can have multi factorial causes. But, there are studies showing the more the brain is used, the less likely having dementia in older age is. So, be sure to challenge your brain-continue learning new things, do puzzles like Sudochi, etc. There are also genetic components which cannot be controlled. Exercise your brain!

This is too broad a subject for this site, but a few comments are salient. First, as we age, our brain atrophies, and this is faster, if we suffer from Alzheimer's disease or Multiple Sclerosis. Stroke prevention by controlling hypertension, diabetes, and hyperlipidemia through exercise, diet, and appropriate medical care also helps. (Predictions are difficult, especially about the future.)