It is something that I've thought about.
I think the younger a relationship starts (within reason), the longer it'll last, as the bond is closer because both (or one) are inexperienced.
The more mature, the more business-like the bond is.
Not to say that in every case it's like that, #notall etc... but yeah, whatever happened to the phrase "childhood sweethearts" anyways? You never hear it said anymore.