I don't know about recruitment age now, I can only speak about 'back in the day'.  As I mentioned, you could sign up if you were 15 but turning 16 in the calendar year - I was enrolled in March, at age 15, and then turned 16 in August of that year, by which time my basic training was already completed.  I would surmise though, that had war been declared, there would not have been any concern about waiting until enrolled Reservists turned 18 before being deployed to the front lines.  (Having said that, this reply is only meant to be explanatory of what was allowed 'back then' - today might be a whole different matter.)