There's a few things you gotta consider. First, while not being the same drugs, Meth and MDMA similarities. They are both methamphetamines, meaning that they affect the same areas of the body, though to varying degrees.
Second, a very large portion (and I would venture to say a majority) of the molly/MDMA you'll find on the street or music festivals is not actually the compound MDMA. It may have traces of MDMA, but its frequently cut with dangerous chemicals. Often, it's not even MDMA but bath salts or synthetic "research" compounds. If you haven't already seen it, I recommend watching the What's In My Baggie documentary.
Third, even if someone has a legit MDMA source, people who use molly are usually using other drugs as well. Those drugs also have an affect and can prematurely age people. Alcohol is a great example.
