maybe a solution could be that bids and sales are disabled until the ages of players are consistent ?
This solution is good, but it has one drawback - that there will be a period with no "trades", and it will be at the offseason, which is the main part where "trades" are usually happens, and in this game this time is shorter than regular.
Having the date of last age-update next to the age-information is a simple way that does not cause anyone any trouble.
Well, except those who are only looking to take an advantage on things that has nothing to do with BB-managing.
"Supprisingly", they are always the same users.
And again, I haven't done that mistake, but I do not want this game to be about anything that is not relevant to what I'm playing this game, and what this game is supposed to be about... BB-managing.
Summarizing, like you, I believe that there is solution, and there is a reason to find a solution to this case.