Well, I was 17 and with a 25 year old and he was useless in every way. Now I'm 24 and with a 20 year old and he's more mature than my ex could ever have been...it really does come down to maturity - and emotional maturity is a difficult one to judge accurately until you really know someone. Besides, despite IQ, people can lose common sense when it comes to what they believe is love - I got an IQ of 145, but was pretty quick to make some bad emotional decisions in the past.
I've said it before and I'll say it again - love is not a feeling, it is an act of the will more than anything else to remain with that person even when the fuzzy feelings fade...because feelings are treacherous and fickle and "follow your heart" is one of the most dangerous phrases to ever be adopted by Hollywood...