Hey don't let age stand in your way.
If you know someone within your network who is maybe a little older, you can do the leg work and be the brains behind the operation, and they can front it.
But hey, don't get too down about being 17, you are already way ahead of the game, just by being here, you have loads of years left to hit this market.
Another thing you could do is approach seo/online marketing companies, be straight about your age and offer to do something for them for free to show them that you are this online marketing whizz kid. As soon as they see your results, they will want to talk more to you. At this point say you want 10-20 of the seo spend for the client, and you will do all the work.
Most ad agencies/marketing companies/pr companies outsource. They ususally pay 50% of the client seo spend to outsourcers, so you never know