Agree it's strange - you have to be "over 18" to buy booze, and we do it legally first time aged 18 and a few hours, not 19. It's probably more correctly "18 and over" but abbreviated for convenience.
Surely you are 70 on your 70th birthday. The next day and thereafter you are over 70. Why should it have to be 71, ie one year later, why not one month later, or one day later?
From government press release:
The proposal is part of a drive by the Ministry of Justice to make the criminal justice system more inclusive and to reflect modern society by giving people aged 70 to 75 the opportunity to serve on a jury.
Currently, only people aged between 18 and 70 are eligible to sit as jurors.