Are there such things as time locked transactions with a single key? Do they send the transaction with a date in the future? What happens if you lose the key the coins are destined for before you reach retirement age?
I have heard that one can make a transaction which will not confirm before a predetermined block. I am not sure about the details, but the coins will be in limbo for the time being, receiving address will be able to spend it when it gets 1 confirmation in the predetermined future.
So, in essence, one would be hoping that the wouldn't lose their private key in 30 or 40 years? That's beside the speculation as to whether or not bitcoin itself will still be around in the future? I know we all want it to be, but storing huge sums with no way to yank it out if something negative occurs, neither of those seem like especially wise decisions...
In theory you can take certain precautions to prevent the loss of your private keys, for example a multisig address that can be signed by multiple brain wallets, I think it would be unlikely that brain wallet farmers would be checking these kinds of brain wallets, and you could keep your keys in your head.
I do agree with the issue of not knowing if bitcoin will still be around by the time someone retires. One feature of most/all retirement accounts is the fact that you have the option to sell an investment, but keep the proceeds in your retirement account if you think it is no longer appropriate to hold such an investment