Chronological age is unimportant (besides very young use, which can damage physical & mental development). I think it's more of a personal thing which depends on your maturity, your health and your life situation. Some people can easily handle drugs and others should never ever touch them.
I did my fair share of herb, blow, speed, cid & etc. back in my late teens/early 20's, then at 25 I got married & started having kids, so I basically cut out everything except a bit of weed and alcohol now & then. Well, actually probably too much alcohol... Time passes - the kids are grown up now & I'm quite a different person than I was back then. For the past 5 years I've been having fun trying all kinds of new substances, but usually not more often than once in a fortnight. I didn't get to try MDMA until I was 46 (wow! what a day!) and I've been doing some amazing explorations with psychedelic RC's. Most of the people I indulge with are 35 or older, and none of us are daily users. We see drugs as recreational or spiritual, depending on the drug. NOT as a lifestyle or daily kind of thing.
I own a small business and I'm quite involved in community affairs. My drug use has had no effect on these areas of life. I don't wear t-shirts with a big leaf, or have any drug related tattoos. I feel no need to publicly advocate for drug legalization or whatever. Again, drugs aren't a way of life but just an addition and enhancement.
On the other hand, I have an old friend about the same age as me and he has really messed up his life with drugs. He's the kind of guy who always has to go farther than anyone else... the last man standing. He's totally screwed up his health, his family and his job. He knows it, but refuses to change. He's on a path of destruction and for him, drugs have not enhanced his life but have destroyed it. This has nothing to do with physical age, but has everything to do with mental maturity.
I know that it won't be too long until I'll be greatly decreasing my use of many substances, especially those that put a strain on my heart. Or I may have to stop earlier if I start loosing control of my usage. But until then I plan on having a good time and exploring my mind. I've got 4 fests lined up this summer and have compiled a good stash to carry me through. Life is good.