I know this theory is hardly original, but when I heard it I really liked it. I'm going to say this now, I'm going to say that the Mother turns out to be the girl from Love Solutions that was a perfect match for Ted, but he threw that away to hook up with Robin.
While anything is possible, the show prides itself on continuity. Love Solutions took place in 2005. The economics class (that we know the Mother was at) took place in 2009, four years later. The St Patrick's Day party with the yellow umbrella takes place in 2008. We aren't sure how old the Mother is, but because she's in college in 2009 (but old enough to be in a club in 2008), chances are she was too young to be using the matchmaking service in 2005.
FWIW, the actress Cristin Milioti is 28, and would have been 20 in 2005 and 24 in 2009. Josh Radnor is 39, but the character of Ted Mosby is 35.
Rachel Bilson (the roommate of the Mother, who Ted briefly dates) is 32 in real life, and Cindy was a graduate student in 2010. Presumably she's close to the Mother's age, since they are roommates.