So, I know there have been a number of posts in the past about this subject, but most of them seem to be closed, and I wanted to share.
Tonight, I had dinner and a long talk with a very old friend of mine, and in the course of the evening we began to talk about my recent breakup with my girlfriend of 6 years. As part of that talk, I told her about being interested in ageplay, diapers, etc. which was one of the things that lead to the breakup.
She was very supportive and accepting, and I can't express how relieved I feel to know that one of my closest friends now knows and accepts me "no matter what" (her words).
The reason I share this is because I never saw a benefit to telling a friend about my ageplay interests in the past. A girlfriend, sure, if it was getting serious. But I always thought telling a friend was unnecessary and could only hurt the relationship. I was wrong...so very wrong. So if you're on the edge about telling a friend, I'd say go for it. It probably won't always turn out this well, but just having someone else on the earth who knows about this has made a world of difference.
P.S. I found this podcast from the Nerdist's Sex Nerd Sandra as well as the big/little podcast VERY helpful in getting ready to talk to my friend. The language that Mako Allen uses and his explanations to Sandra, who doesn't live with this every day like we do, resonated with me and were quite useful in explaining to my friend. (wish this had been around when I came out to my girlfriend a couple years ago...oh well)