The age of a study in and of itself tells us nothing about the validity of its conclusions. Only if a newer study uses a larger dataset, or explicitly addresses deficiencies in an older study's methodology, can it be presumed superior to the older study.
First of all, the marijuana of today contains more TCH than the old marijuana and the studies that are being done today use diagnostic techniques that were not available in the early seventies.
New studies show that there is real observable physical damage to the brain of marijuana users. Marijuana is stored in the fatty tissue of the body, the brain is the largest depository. The damage can be seen on an MRI.
Other studies show that ingested marijuana is more powerful than smoked marijuana. There have been two deaths in Colorado that have been attributed to marijuana laced food, one a cookie, the other, candy.