Nice description Roi G... copied word for word from the urban dictionary though
See here: http://www.urbandictionary.com/define.php?term=grab+a+granny
Lots of interesting stuff on this webpage, A night at the museum (see 3rd defintion on page above) was a a new one for me:-)
one important thing about the phrase is that age is relative to the person looking. If the person looking for a 'partner' is 20 then a 30 year old woman would count as a 'granny' even though in reality she is very unlikely to be a grandmother or look like one.