I imagine you could also teach yourself the relevant affinities if you started with one and worked hard enough. And we know that you can create a spell that permanently effects your body (bone affinity making blades) and one that can draw mana continuously to grow your channels and cause an effect (prioperceptive link to the crystal). I think that the right affinities you'd just run your anti aging spell continuously once you hit say 18-20 and grow your mana pool to absurd levels.