Assuming a modicum of familiarity with railfan topics one usually can tell a poster's age by the questions they ask/statements posted and their structure, while not a rule of thumb generally the poster's ability to co-ordinate the topics with their relevant dates/history is a dead give-away, questions like " did the Santa Fe ever buy gallery commuter cars"- a reasonable question if the poster was unfamiliar with the AT&SF pre-Amtrak. Not to imply there aren't young posters out there who don't know their stuff, far from from it, just that time usually tells, example- I'm near 50 but largely ignorant about steam technology. Generally younger posters will congregate where they're accepted and they can contribute without ridicule. The second "tell" is sentence structure, typically different generations use different syntax and grammer, web-contractions, e-slang, aol shorthand while not always a give-away usually will tell you whether the person your dealing grew up with B&W TV or a Gameboy. My .02 the best rule of thumb to encourage posters of all ages is to assume that all you respond to are made by friends, neighbors or relatives, that is; the old "Golden Rule".