In this politically and socially charged area... I am not sure that funding will be made available for research into the effects of marijuana on older brains.
My wife is a retired nurse and I of course worked as a fire officer in a large city during my career. We both had a lot of opportunities to see first hand the results of many years of marijuana use on people we helped treat in addition to family and friends.
It is interesting how little the public including users actually know about long term negative effects that many health care professionals are aware of. One symptom of long term use that my wife and I both noted many times were severe varicose veins in people at an earlier age than one would normally expect to find them.
My wife's ex-husband was a heavy user at an early age and had horrible varicose veins starting in his early 20s. His doctor blamed the marijuana. I have two sister-in-laws who also have been heavy users who started suffering from varicose veins in their 20s. None of them have family histories that would suggest problems with vascular weakness. What they have in common is heavy marijuana use from an early age.
But much more serious than the unsightly appearance and discomfort of their varicose veins is the increased risk they indicate for developing deep vein thrombosis (blood clots cutting off circulation) and strokes. These are all conditions associated with vascular health and they are all conditions that have long been thought by doctors to be associated with habitual marijuana use.
When a person has a stroke in their 20s through 40s they are very likely to be found to be habitual marijuana users. By the time we reach our 50s and beyond the chances of a stroke from other causes increases but it does not mean that marijuana has not played a role as well. Marijuana has been shown in studies to cause changes in the circulation in people and animals brains irregardless of their age. But one of my chief concerns would be indications that habitual use is associated with poor vascular health that affects the brain and other vital organs.