You have to take in account also local customs and jurisdictions: age to travel alone can vary between 12 and 18 , there are differences also on minimum age for registering to an hospex service (in Europe is 13 to 16 according countries). For example here minumum age to register to service is 14, while for traveling alone can be 14 (with implicit or explicit permission - explicit means a written authorization to go in a certain place to a certain person, implicit is just the fact that the subject can freely show its passport), 16 (without explicit veto). other eu countries have slightly different limits but I would keep 16 as a general rule.
About pelgrim’s question: this is the reason it is advisable that anyone under 18 send its personal request, and not together with an adult accompanying, expecially if adults are parents or siblings: a request to host that came from parents has the value of an “explicit” authorization to travel while if the request come from the person itself you have just to chek on arrival that has a valid passport and that it shows that the bearer is 14 or more, so it has the implicit authorization.
The fundamental difference ifs that an explicit authorization is addressed to an host and pass to the host some obligations that the parents would have (among them: feeding the person, caring of it including if necessary new clothes and assuring a return trip, not having consensual sexual contacts if the guest is under 16) while with implicit authorization you may treat the over 14 guest as an adult, so you do not have any specific obligation or prohibition, other than the normal ones.