It has nothing to do with age. if it cant be cracked it cant be cracked. choose 12 words at random from a small dictionary, write em' down somewhere safe and secret, and forget about it. That will work forever. Overkill security is better than trying to find the computational limit and then going one bit above that. keys are cracked when the process of generating them is flawed or the key is not big enough. That's why bitcoin private keys are so long and why 12 words are used even though 8 would probably still be enough.

Cool - thanks.