There is a large range of ages that go planting. One of my foreman's sons came out for a bit of planting when he was 15. He didn't become a pounder or anything, but he was fully able to plant. On the other side of the spectrum, I've had 40 some odd year old rookies come out and stick with it for 4 or so years, making good coin. As for the former, it really comes down to availability. You really need to be done highschool in order to get a job in the planting industry here in Canada. Across the country the regular planting season starts up as the snow melts (late april/early may). and companies don't really do much hiring into the spring season.
In British Columbia the planting season goes as such: Late Feb the coast (mostly low lying Vancouver Island) starts up and goes till early April. Late April/early May the interior starts up... this is the meat and potatoes of BC planting, it runs for two or so months, typically ending in early July. from mid July to mid August it's too hot and dry in BC to plant so everything shuts down... a lot of people head out to Alberta to keep working into August during this time. Myself I take this time to go on a trip somewhere cool. In mid to lake August the coast starts back up and we plant on the coast till mid October.
Coaster planting is what separates the boys from the men, it's very rare to get a job on a coastal crew as a rookie. As for good companies, all the really good ones won't hire rookies due to the state of the industry at the moment. I'd say Brinkman is your best bet. none of their camps are bad, some are better than others yes, but none are bad. And as for how much you make, it's really up to you... I'd say in your first season if you do a three month season (BC for may/june and Alberta for july) you should make 5-10g but expect it to be closer to 5. Last year I only ended up working 3 months instead of my usual 4 and walked away with 20g. combine that with EI and I had the entire winter off to ski and party with no money worries.
It's a good way of making money in the summer and having your winters off... but it's not the only way. Forest fire fighting is worth looking into, as well as the sport fishing world, and many other options - i'd keep going on and on, but I have to go skiing now. peace.