Ugh, its a typo in the achievement text. It should have said "combos", which is when you match three or more to the next age group up.
Thanks for letting me know.
Ah cool, I've also noticed the 'five dollar cheque' achievement doesn't fit in the bubble with the date next to it.
In other news, the game is (sadly) much more straight forward when you switch to number mode. Maybe that's just how my brain works.
