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)
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?