Getting H4 for the stepchild before it completes 18th birthday

[h1bstudy]

I have the following question.

The Indian national X has been on H1B for more than 6 years (The I140 is approved). Soon X is planning to marry another Indian national Y who is residing in India. Y has a kid Z (also Indian national) who is also residing in India. The kid is not yet 18 years old (should be completing 18 in a few months). But X can't marry Y right away because Y has filed for divorce (mutual consent) in India from its existing marriage which should materialize in 3-4 months (best case scenario) I guess.  The question is -- if , for some reason the divorce takes time for Y and then X ends up marrying Y after Z (the stepchild) crosses its 18th birthday ,  can X still get Z on a dependent visa (H4) to USA? I read on some forums that only if X marries Y before Z (biological kid of Y from prior marriage) completes its 18th birthday , only in that case Z can get dependent (H4) visa. The custody of the child is not the question here , because at the time of travelling internationally Z will be 18+.  The goal for X is to get X & Z to live with him in the US. I know that even if Z gets H4 , it will only be up to 21 years of age. That's fine

[newacct]

A stepchild is only considered a child for US immigration purposes if the stepparent married the parent before the child turns 18. So if they marry after the child turns 18, the child will not be able to get H4.