As far as Alexander's first appearance goes, when K'Ehleyr first appeared, it seemed Worf had already had a previous relationship with her, so, I assumed she already had Alexander from that previous relationship, and just didn't bother telling Worf about him? So, I never questioned him being about6 on his first appearance.
But, yea, Teenager Alexander was definitely a case of Soap Opera Rapid Aging Syndrome (SORAS)
It was strongly implied by the two of them that their encounter during that episode was the first time they'd had sex. After it's over K'Ehleyr asks why didn't they do this a long time ago and Worf spouts off about honor and being too young. But that is definitely one explanation of his age, that he was already around, I just don't think the "evidence" backs it up.