OK, I'm 26 and living with my parents (though in my defence I did live away form home for the best part of 6 years) and I recently put myself through my CBT and DAS then bought a bike.
My Mum was never going approve, especially with a bunch of 'ped-owning organ-donors living just around the corner) so had to take the 'responsible adult' tack.
First of all I paid for the whole lot myself. Must have forked out about £2k all-in-all. This is money I'd saved up myself for this expressed purpose.
Secondly, to show that I was taking my safety seriously I made sure that I got decent leathers. I wound up spending about as much on leathers as I did on the training itself, but all in all, it's gone a long way towards assuring both my parents, and my somewhat sceptical friends, that I'm not about to wrap myself around the first available tree. Oh, and don't let any organ-donor mates you might have give you a digging about not riding around in shorts and t-shirt on blisteringly hot summers days. For good leathers this isn't a problem as they breath when you're moving along, but more importantly chicks dig guys in leathers
Thirdly, whilst I had my heart set on getting a 535 Virago as my first bike I instead opted for the somewhat more sensible Honda CB500S that is my current steed. So don't insist that you want your first bike to be some ludicrously fast 600cc sports bike aim for something sensible to build confidence and skills up first before progressing on to something more wild... personally, I'm aiming for my next bike to be a Ducati 749.
So all in all the best advice I can give you is to be sensible about the whole thing. Be calm and collected (throwing a tantrum about not being allowed to ride won't do any you favours) and above all, show that you're serious about being sensible and that it's something you're determined to do even without their help. At that point they can either help out be giving contributions towards proper safety gear and training, or let you get on with it and risk you taking cheaper shortcuts

By Bike.
By Car.
By Truck.
By Foot (if desperate).
Never by Bus.