A couple of points to note...
Hi there, good on ya for giving this a go; you never know where it might lead to.
I did see website the other night where some guy was charging about $80USD for painting and decalling - I dunno if those prices are still current.
First thing is, learn how to take excellent photographs of your work - I'm not trying to be funny or be mean, but the sample photo's you posted were very underexposed and grainy and it was hard to see how good the paintwork really is. If you're going to showcase your work a good photograph speaks volumes. Look at photos in different publications and get a feel for how they are set up and composed. It will be a learning curve, but worth it.
Secondly, make it very clear to the client (and that's exactly what they are) what they can expect and make sure you know what they are wanting - there's nothing worse than having to re-do something because of miscommunication. Even if you set up a standard form to fill out and email or mail to you with basic and specific details - road name and number, is it to be weathered, etc.
As you start making some money from painting, invest some of it back into good equipment - a decent airbrush and compressor, lighting etc - all of these will help improve the next job you do.
Always be critical of your own work and see what areas you can improve on. You've gotta be satisfied that you've done the best you can (within the constrainst of time and bbudget of course!)
Be realistic with your turnaround times - real life has a habit of intruding into activities like these; people get a bit grumpy when their order is delayed.
OK, that's all from me - best of luck with giving it a go. I wouldn't worry too much about your age, so long as you have all your bases covered adequately.
Best regards from NZ
Mike A