I told them about a month ago. During the last year or two I've noticed that my relationship with my parents has changed to be closer and more honest, more "adult-like". I'm 33, married, have a 5-month-old baby girl, a job, house.. and have always been responsible and centered. I would say I'm a good person and a good son, and they think the same.
Since drugs have been a big part of my life for about 15 years I decided to come clean and tell them all about it. Let's say that I didn't want secrets between us anymore. I've tripped hundreds of times over the years but have slowed down my use quite a bit. Nowadays I don't even drink but often do psychedelics, dissos, entactogens and smoke weed, and have a stash large enough to last me a lifetime of sporadic use.
They didn't have a clue and it came as a complete surprise. I explained how I'm responsible with my use, how I study the substances, take care about the setting, my own personal rules (no IV, IM or intranasally, low doses, no hardcore shit (heroin etc)) and how it's something like a hobby.
My dad is a doctor and my mom a nurse and that night I learned that they have never done any drug. At first he was a bit angry about it cause he has seen what drugs do to people but they accepted it after taking for a while. Since I've never had any kind of problem with the law, work, studies, etc.. they just had to accept that I am a responsible user. They also appreciated a lot that I could confide in them and just asked me to not be fucked up around my daughter.
One of the reasons why I wanted them to know it, is in case that something really fucked happens in my life. For example, if my wife and daughter die... I would probably get in trouble with substance abuse. I prefer them to know that it is a possibility so they can help me if I need help. Same thing if the police stop me while driving and drug test me, I don't drive under the influence of anything but I'll give a positive for weed and benzos for sure because they can be detected days after use. It's better for them to know that I use stuff beforehand than getting double-shocked.
All-in-all, I recommend to tell them if you are an adult, they are at least a little bit open-minded and if you think like I did, that you would like them to know. The next step is to encourage them to take something and have a nice experience, maybe some MDMA or at least smoke some weed with them. Little steps...