I don't really see why age limits would be exempt for "voters need to take responsibility for their voting".

IMO, the real issue is that voters are coerced to accept candidates put up by the parties due to FPTP. The threat of the wrong side winning gets people to accept someone they don't want. The primary process does not need to be democratic, and the results are pressured by the future threat of losing to the other side in a head-to-head.