That's the thing - birth dates won't matter any more -this was also one of the arguments against a change - those in their 40's and 50's probably wouldn't want citizenship or move to the UK at this point in their lives anyway.
But, seriously, if someone in their 90's who is the child of a British man wanted to, at this late date, apply for a British passport after having been born in Florida and lived there all his/her life, they could if they wanted to...the same goes for axe murderers and serial killers, apparently, because all they have to do is apply for the passport (that is, as long as their British dad was actually married to the mother!).