its more about the pointless rule making than the actual age, set it to 25+, now someone comes along who is 24 and plays well and gets on with everyone and we don't let them in? Its ridiculous. Recruitment needs a policy I agree, but it should be based on a first and a second from within the guild, otherwise its just random chance anyway and some age limit is fine, but the game already imposes one anyway.
fewer rules means more time to do the things we want, unless enforcing rules (which are unenforceable anyway, if someone is 14 and wants in they will lie and no one will know) is what you want to do.
I would say 21+ for spam recruitment, no restriction if recommended from within guild.