Ill tell you my experiences...
I am 17 now for the record. As early as 13, I hung out in the shop during the summers, winters... all the time. I got to know the guys, I asked if I could help out in the fall, at age 14. They were chill and trained me for rentals right away... I worked for free for about 6 months but have been working there for pay ever since. Im still a rental bitch, but it is totally worth it... Cheap gear, great people, great parties... Ive been trained fairly well in the repair shop and done quite a bit of retail sales. Its been a great experience and I imagine it will give me a leg up in the industry in the coming years... Do what ever you can to get the job, but dont be dissapointed if you are doing rental`s a few hours per day... Then just take every opportunity to learn repair shit and you are golden...
Good luck, and just remember its all for the love of the sport.