Just Googled this for interest. In 2011, a Romanian gypsy became a grandmother at twenty-three. The youngest authenticated on record was seventeen.
I guess in the mists of time when sex wasn't even recognised as a factor in reproduction and then, later, when biological rather than social or cultural restrains was still the overriding factor, even younger grandmothers would have been around.
Found this info and more here if you want to take a look:
http://webcache.googleusercontent.co...www.google.com
Cheers to all grandmums. Hope you're as happy as I am as a grandfather four times over. Neil